First, let's start with the basis for my assessment: I've played the game for about 120 hours and I did all the main quests, all the side quests and all the contracts. I explored almost every bit of the world, fought against every monster in the game, met all the more or less important characters. I've experienced two of the main endings as well, by changing some of my decisions in the game. So I'd say that I pretty much know of what I'm speaking which doesn't mean that I see my points as ultimate facts that cannot be denied but that my points are based on actual comprehensive first hand experience and the corresponding thought process.
Second, good things first: for most of the time, the game is pretty much awesome, almost perfect if you ask me. I've never seen a game world that seemed that alive and good looking and just believable in its own world rules and limitations. There are so much dynamic elements in this game, so much attention to detail, so many awesome, little moments in which you think "that's it". Other (partially) comparable games like Skyrim or Dragon Age Inquisition are one or two levels behind the immersion and believability this world offers. The narrative is naturally quite important for that feeling of immersion and believabilty, besides the more technical stuff like graphics, sound and animation. And for the biggest part, the writers did an awesome, even outstanding job. Especially many side quests and some of the monster contracts feel well written, well executed and mechanically diverse (in the limits of the genre) and satisfying, a combination which is quite rare in gaming. Based on world building, overall narrative approach (most of the time), atmosphere, technical aspects and basic gameplay alone, the game would surely deserve the highest ratings, I'd say even a 10/10 (especially when we look at the ratings other RPGs got in the past...)
There is a big BUT though: there is one specific element of the game that doesn't hold up to the rest of the game's mechanics and elements, namely the main narrative, especially in the later parts of the game and at the very end. While the game's main narrative is quite enjoyable, fascinating, entertaining and last but not least logical and well designed and staged it turns into a bad hot mess in the last third of the game. I will explain in the following parts why I think so and which elements contribute to that assessment. I will start with the pacing, story structure and overall storyteling approach. Then I will evaluate the choice&consequence mechanics and how they influence the ending of the game and the epilogues. And finally I will put some light on the writing and design of main characters of the game and their contribution and their depiction in the later parts of the game in particular.
Edit: I've extended the analysis with a fifth chapter about Ciri and her abilities and whether she qualifies to be the saviour of the world and the defeater of the White Frost or not. Since this post would have become longer than the allowed maximum of words for a single forum post here (beat that, guys! ) I had to put it in another post that can be found here.
Edit 2: I've again extended the analysis with another chapter on choice and player agency. It can be found here.
1.) Pacing, story structure and overall storytelling
There is a certain, "optimal" pattern for how to pace a good and gripping story in writing theory. Star Wars Episode IV is for example a quite common and rather famous example for an almost perfect story pacing. It looks like that: