tl;dr : Wells its co-op online Fallout, with more or less what you'd expect. PVP is not within your control despite the systems that are supposed to allow that, and its a janky random spammy mess with awful shooting mechanics that can't keep up. The jankiness in the rest of the game is more or less within the acceptable ranges and norms for Bethesda, which isn't ideal but is passable. Of serious serious note, they need desperately to clarify how persistency and servers and all that work.
UI, to start simple. As might be expected its not great. The pip-boy (inventory and quests), the map, the build mode and the crafting stations all have different UI's with different sets of controls and no particular unifying design to them. And the trading UI is just a confusing mess, that my group never did figure out (ended up just dropping things for each other). What sort of worked in singleplayer Fallout also cracks pretty bad in real time. Trying to pop open an inventory and jam a buff item or switch to a weapon is laborious, and the Favorite wheel is its own mess (its not hold to toggle, and also requires press to confirm, in contrast to the usual standard). Quests also deserve a special mention, all your quests (and your co-op team leader's quests) keep piling and piling on the top right in a giant mess. You can finagle through the pip boy cludgy interface to remove them, but any new events or whatever else will keep adding on. And the quest markers show up on the compass, but without any noticeable distinctions to them, so if you don't actually clean up that list, that navigational aid is basically unusable.
The setting is kind of "Eh...". You'd think with the no NPC thing that you'd be getting a dead wasteland to explore, which would be intriguing enough its own right. But you don't really. Everyone is dead before you find them, but its fairly recent deaths. There's even Responder (sort of the Minutemen/NCR esque group of the area) Protectrons randomly about still. With the world so populated, the concept of "All the people are players) is less of a serious design philosophy, and almost seems like some tongue-in-cheek satire. There's also air drops, and Responder Alerts (public event MMO esque things) that would thoroughly indicate some sort of living presence.
The combats more or less serviceable. Its hard to tell if the shootings off, or some sort of latency is going, but with any faster moving enemy you're much encouraged to use VATS which is auto-aiming mode in this real time variant. Long rifle range seems weirdly short, whether that's a lag, or a design choice I don't know, its very very noticeable when you're trying to target a cargopod or one of the showpiece enemies that is a flying creature. AS the enemies go, there is a fair variety, possibly more then any of the other Bethesda Fallouts. Raider's do effectively exist, but they're "Scorched", victims of some kind of plague, and largely resemble ghouls (but aren't radioactive, and do use guns), more kind of wishy-washy adherence to that "no people" bit.
Base-building!. Not actually completely terrible. Grindy as hell, but thats the bread and butter of these survivally game things. The UI is a little confusing as mentioned. But fairly versatile. One quibble is that if you're setting up around existing structures, it doesn't play nice at all. So your claimed workshop/outpost thing is a little rough. Some things you can move or remove, others you can't, sometimes you remove a desk (for instance) and the rubble effect on top of it remains floating in the air. Wall/Ceiling mounted stuff will only attach to your built walls, not pre-existing stuff. You can build using your (singular) camp claim anywhere you want outside of a claimed outpost as well, but its a much smaller radius, and obviously lacks some of that post-apocalypse authenticity of setting up in an abandoned factory or the like (also the reclaimed sites tend to have some perk like junk piles that you can put an extractor on, or existing power supplies, etc (besides junk piles, our home factory had machines that kept refilling with packages of mac and cheese)).
On a distinctly concerning note however, as people randomly disconnected on our squad of 4, ownership kept shifting erratically over claimed sites. Also only the owner can seemingly fast travel to a site, rather then the whole team. This raises (along with their non-existence of a traditional server setup), quite a lot of questions about what happens to your stuff when you log off.
PvP. According to theory, you won't be in PvP unless you shoot back at someone. They'll do 1h per hit on you until then. Right off the gate, the obvious loophole in that isn't sealed, someone can just run in front of your bullets until you hit them, then fire back themselves whenever they feel like it (a fellow rather enthusiastically beating me with a pickaxe to no effect while I pulled out a shotgun, took psycho, then fired back, for instance). In pitched firefights (like those events I mentioned), you'll potentially end up in unwanted PvP mode. The promised "passive mode" style option they backpedaled about doesn't appear to be in the game at this point.
Speaking of unintended PvP. Bases are broken as hell. Turrets have an absolute mind of their own, attacking some people, ignoring others (sometimes even while they're invading your base). We actually had two guys run into our base, be ignored by the turrets, claim it, then the turrets started attacking us. Also you can kill invaders, but they just respawn nearby with all their things and can run right back in (they're even incentivized by a revenge bounty that gives caps for killing someone who attacked/killed you.). So base battle is just an endless war between perpetually respawning people. For that matter the aggro mechanics were kind of odd, one invader was toggled for PvP, the other wasn't.
And if the shooting was kind of dodgy for NPC combat, it just absolutely loses its mind in PvP. Hits don't register constantly, and the minute a shootout between players starts it seems to drop frames and/or lag like crazy. Also, as a minor note, outfits are basically skins (they weigh 0.1), that also cover any armour the player is wearing, which is a notable flaw in terms of picking engagements.
By and large, if you want the Fallout survival mode co-op experience, its probably there. But without private sessions as an option, the annoying awkward PvP (which is subpar even to questionable combat in things like Rust and Ark). Even without that particular problem, sharing the map with randoms can be obnoxious as building sites aren't instanced, so a claimed site can't be used. (And I have no idea what happens if you join a session where someone else has claimed your site).
UI, to start simple. As might be expected its not great. The pip-boy (inventory and quests), the map, the build mode and the crafting stations all have different UI's with different sets of controls and no particular unifying design to them. And the trading UI is just a confusing mess, that my group never did figure out (ended up just dropping things for each other). What sort of worked in singleplayer Fallout also cracks pretty bad in real time. Trying to pop open an inventory and jam a buff item or switch to a weapon is laborious, and the Favorite wheel is its own mess (its not hold to toggle, and also requires press to confirm, in contrast to the usual standard). Quests also deserve a special mention, all your quests (and your co-op team leader's quests) keep piling and piling on the top right in a giant mess. You can finagle through the pip boy cludgy interface to remove them, but any new events or whatever else will keep adding on. And the quest markers show up on the compass, but without any noticeable distinctions to them, so if you don't actually clean up that list, that navigational aid is basically unusable.
The setting is kind of "Eh...". You'd think with the no NPC thing that you'd be getting a dead wasteland to explore, which would be intriguing enough its own right. But you don't really. Everyone is dead before you find them, but its fairly recent deaths. There's even Responder (sort of the Minutemen/NCR esque group of the area) Protectrons randomly about still. With the world so populated, the concept of "All the people are players) is less of a serious design philosophy, and almost seems like some tongue-in-cheek satire. There's also air drops, and Responder Alerts (public event MMO esque things) that would thoroughly indicate some sort of living presence.
The combats more or less serviceable. Its hard to tell if the shootings off, or some sort of latency is going, but with any faster moving enemy you're much encouraged to use VATS which is auto-aiming mode in this real time variant. Long rifle range seems weirdly short, whether that's a lag, or a design choice I don't know, its very very noticeable when you're trying to target a cargopod or one of the showpiece enemies that is a flying creature. AS the enemies go, there is a fair variety, possibly more then any of the other Bethesda Fallouts. Raider's do effectively exist, but they're "Scorched", victims of some kind of plague, and largely resemble ghouls (but aren't radioactive, and do use guns), more kind of wishy-washy adherence to that "no people" bit.
Base-building!. Not actually completely terrible. Grindy as hell, but thats the bread and butter of these survivally game things. The UI is a little confusing as mentioned. But fairly versatile. One quibble is that if you're setting up around existing structures, it doesn't play nice at all. So your claimed workshop/outpost thing is a little rough. Some things you can move or remove, others you can't, sometimes you remove a desk (for instance) and the rubble effect on top of it remains floating in the air. Wall/Ceiling mounted stuff will only attach to your built walls, not pre-existing stuff. You can build using your (singular) camp claim anywhere you want outside of a claimed outpost as well, but its a much smaller radius, and obviously lacks some of that post-apocalypse authenticity of setting up in an abandoned factory or the like (also the reclaimed sites tend to have some perk like junk piles that you can put an extractor on, or existing power supplies, etc (besides junk piles, our home factory had machines that kept refilling with packages of mac and cheese)).
On a distinctly concerning note however, as people randomly disconnected on our squad of 4, ownership kept shifting erratically over claimed sites. Also only the owner can seemingly fast travel to a site, rather then the whole team. This raises (along with their non-existence of a traditional server setup), quite a lot of questions about what happens to your stuff when you log off.
PvP. According to theory, you won't be in PvP unless you shoot back at someone. They'll do 1h per hit on you until then. Right off the gate, the obvious loophole in that isn't sealed, someone can just run in front of your bullets until you hit them, then fire back themselves whenever they feel like it (a fellow rather enthusiastically beating me with a pickaxe to no effect while I pulled out a shotgun, took psycho, then fired back, for instance). In pitched firefights (like those events I mentioned), you'll potentially end up in unwanted PvP mode. The promised "passive mode" style option they backpedaled about doesn't appear to be in the game at this point.
Speaking of unintended PvP. Bases are broken as hell. Turrets have an absolute mind of their own, attacking some people, ignoring others (sometimes even while they're invading your base). We actually had two guys run into our base, be ignored by the turrets, claim it, then the turrets started attacking us. Also you can kill invaders, but they just respawn nearby with all their things and can run right back in (they're even incentivized by a revenge bounty that gives caps for killing someone who attacked/killed you.). So base battle is just an endless war between perpetually respawning people. For that matter the aggro mechanics were kind of odd, one invader was toggled for PvP, the other wasn't.
And if the shooting was kind of dodgy for NPC combat, it just absolutely loses its mind in PvP. Hits don't register constantly, and the minute a shootout between players starts it seems to drop frames and/or lag like crazy. Also, as a minor note, outfits are basically skins (they weigh 0.1), that also cover any armour the player is wearing, which is a notable flaw in terms of picking engagements.
By and large, if you want the Fallout survival mode co-op experience, its probably there. But without private sessions as an option, the annoying awkward PvP (which is subpar even to questionable combat in things like Rust and Ark). Even without that particular problem, sharing the map with randoms can be obnoxious as building sites aren't instanced, so a claimed site can't be used. (And I have no idea what happens if you join a session where someone else has claimed your site).