It's... ok. Nothing fantastic. But enjoyable.
Played the Closed Beta, currently playing duo squad with a friend. I find it does enough to keep me interested, but definitely not enough to actually have me enthralled. I imagine this is what WoW players must feel like, invested enough to keep going but subconsciously aware that you're not having much fun.
Let me be clear, this IS an MMO. Mob spawns, loot grinds, stat min/maxing, it has all the hallmarks of a skinnerbox-style MMO. Friend even claimed he felt like he was playing Diablo.
Personally, I feel gameplay is a lot more like Borderlands than Diablo, with more focus on cover-based tactics than run-and-gun. If you liked Borderlands (in particular the new game+ grinding) then this will certainly interest you.
Basic gameplay is cover-based shooting and... that's it. Shoot at enemies, take them down, move on. It does get monotonous, but mechanically speaking it's actually pretty satisfying. Since all enemies are bullet sponges and take a few seconds to kill, you have to actually think about how you approach a fight; can't just run in dropping thugs left and right. Instead you have to prioritise targets, pull flank manoeuvres, and generally approach gunfights with a tactical mindset.
Story is pretty decent as well. Basic premise is that of a virus epidemic, with paper currency being the transmission vector. People get sick, people panic, people go crazy. To me it feels like classic Tom Clancy material, and the plausibility of it (at least compared to T-Virus zombies) is a welcome change.
One thing I really do enjoy is the Dark Zones. These are your PvP areas, with players bumping into each other while searching for rarer-than-normal loot. The idea is that it's the same as PvE, but friendly fire is turned on; so it's totally possible to kill other people, take their stuff and run for the hills. Doing this puts a big red target on your head, encouraging others to hunt you down.
I had the most fun in ages during Beta, involving an innocent victim I murdered, 3 PC squads on my tail, and me running for my life in a 5 minute chase through the alleyways of New York. Felt like Harrison Ford in The Fugitive. This aspect of the game is probably the part that will keep players hooked, although as it stands it does lack an overarching goal. Currently it's just grind for loot, watch out for bandits, rinse repeat.
TLDR; Mediocre, but servicable. Combat is actually engaging (because enemies actually take thought-process to kill), story is decent (if uninspired), and PvP can be a barrel of laughs if you're willing to murder (but overall is kinda pointless).