It's a landmark game for sure. The dialogue is excellent, the story and the world are full of rich details, and the quest design is highly creative. The side-quests in particular are some of the best and most intriguing I've ever played in an RPG, and the main story overall is great too, wrapping up the series nicely. The writing is mature, intelligent, and treats the player as an adult, but there are good light moments in there too; good humour and fun situations to mix things up. Moral choices are all difficult, all shades of grey, and it's rare you'll feel like you made a definite "good" decision. The combat isn't very deep, but the reliance on preparation, the necessity of using the right potions, oils and bombs, and the frankly insane variety of enemy types, make it all more engaging than it has any right to be.
Let me hit the game with some criticisms though... It's a shame that potions and oils don't work the same way as they did in the other two games. Before, you needed to brew them for each use, but they would last a long time. Here they replenish automatically and only last a short amount of time, which makes them seem much more disposable and makes planning for each encounter less of an intricate and important process than it used to be. The skill points system is strange. You can unlock lots of skills, but you can only use a small number of them at once, which feels very limiting. The difficulty is inconsistent, and I found myself often raising and lowering the difficulty to try to find a balance that just doesn't exist. Your horse is an idiot. She is so annoying to control because her movements are unpredictable, she can get stuck on nothingness, and will frequently bug out. She'll also spawn in the most awkward places when you call for her, and then won't be able to get to you because her pathfinding is atrocious. Geralt controls awkwardly as well.
The game world is very impressive. It's varied and interesting, everything fits and makes sense, there are so many things to do, and it feels alive. It's beautiful too, not just technically, but artistically, and only let down slightly by the lack of variety in NPC faces and the fact that it rains all the damn time. It's a new benchmark for the open world RPG that other games are starting to copy, but at the same time, it can't avoid the pitfalls that all of these type of games fall into...
The side quests, while great, also ruin the pacing and the urgency of the main story, because they're everywhere and take you on leisurely detours all over the map. Since there's no oversight for which enemies you'll face at what time, simple bandit enemies can look and act identical the lower level bandits, but be at such a higher level than you that they can kill you with two strikes, while you can barely scratch them, which makes Geralt seem suddenly weak for no reason. A bigger world also means a bigger focus on loot, and you'll eventually be discarding legendary loot like it's junk, because you've already picked up so much, you don't need more money, and all rewards are now pointless... At one point I was gifted a sword by a major character, and there was a long cutscene which really built up the quality of this sword. Once I got back into gameplay I checked my inventory, and found it to be considerably worse than the sword I'd been using for the past 6 hours (and probably found in a bin somewhere.) Also, why are there enemies literally everywhere I go? Can no one travel for five minutes in this universe without coming across bandits, drowners, arachas, or whatever? How is anyone brave enough to leave the house, and how have people even survived this long? These kind of things really hurt my immersion and lessen the impact of everything, and the other Witcher games didn't have these issues.
The other games in the series, though they were open-ended, had smaller and more confined areas, with a somewhat more linear progression through them. They gave you freedom to do what you want, but they were always pointing you in the right direction and keeping you on track, so they were a lot more deliberately paced. The pacing and progression was excellently done in both the previous games. Not so much in this one.
So I don't think it's quite the masterpiece that some people have made it out to be. Like the other Witcher games, it's great, sometimes superb, but has notable flaws. After over 100 hours in this game though some of my criticisms feel like nitpicking, because it captivated me in a way that few games ever have. It's not perfect, but it is unforgettable.