So I'm about 15 hours in, and I'm going to try and not wax too poetic about this game, but...
I seriously can't remember the last time I enjoyed a game this much.
Any issues I have with the game are tiny little nitpicks compared to the game overall.
-The graphics are incredible (even on low settings), and the little details really sell you on the world. The swaying of the trees in the wind, the way Peasants hide under roofs during rainstorms, even something as simple as hearing Geralt's boots slosh through mud in the village streets really sells you on the entire world.
-I read every book, note, and character description I pick up in the game to get more detail about the world I'm in. I don't think I've done that since Baldur's Gate 2 and Planescape: Torment.
-The game sells its dark setting incredibly well without having to resort to the usual over the top antics of having excessive amounts of cursing, or rape, or pillaging, or other things just to beat you over the head with how super depressing everything is. It feels like a dark ages game: the peasants aren't bad people, but they're superstitious and uneducated. The Lords aren't necessarily bad people, but they have to rule with an iron fist to keep order. I always use this as an example: whenever I complete a contract to kill a Monster, I have to genuinely search my conscious if I want to take the money (which I very much need) or let these poor peasants keep it, because I genuinely feel bad for their struggles. If that's not immersion, I don't know what is.
-Monster hunting actually feels like, well, hunting. It's not just "go over to this cave and kill this monster". You actually investigate the site of the attacks (or talk to locals) to find out what you're dealing with, hunt down where the monster is located, and then prep for the battle depending on what sort of monster you're fighting. Monsters (both big and small) genuinely feel like you need to put strategic thought into how you're going to fight them and prepare appropriately.
-The characters are, so far, outstanding. Geralt is 1000x better of a character than he was in the previous games. Hysterically, even when you get sent on really mundane quests, he actually reacts realistically ('you want me to retrieve your goat? Seriously?'). Characters who you only work with for a small chunk of the game are better all around than some characters you play through the entirety of Dragon Age: Inquisition with (Vesemir and Keira are excellent, and I'd argue the Bloody Baron is support character of the year so far). I'm looking forward to seeing who else the game has to offer.
Ok, enough praising, but flat-out: if you like RPGs, I really can't recommend Witcher 3 enough. Witcher 1 and 2 had some really good ideas but they just couldn't quite get the execution right. This game (so far) has taken those good ideas and nailed the execution, it's outstanding.