Just finished the game. Really, REALLY good ending. As I said before, the strength of this game compared to any other RPG I've played is the story and the depth of the characters. Very well done.
Ruminating, I think part of this is due to fact that you can only play a single character, Geralt. Unlike, say, Skyrim, where there are hundreds of possible race+class combinations. Granted, it makes Skyrim a bit more re-playable (still not sure I'd want to replay Witcher 3, maybe someday...), but that forces the main story to be a bit more generic, so that it could fit any of those characters. Witcher's story is perhaps more narrow, but more deep because of it. There are still choices to make along the way, and some of the little epilogue snippets I think are directly determined by conversational choices earlier on, which is a nice touch.
Anyway, ending boss fights (two of them) were not as hard, for me, as the first major Wild Hunt general (Imlerith). Still pretty challenging though. Maybe because I was relatively higher in level? Not sure. (Ended at level 34.)
Overall I really liked the gameplay. Good magic system, lots of loot to be found, plenty of side missions and a much more open world than the previous Witcher installments. PC version was a little unstable, crashed every now and then, hopefully those'll be fixed with some future patches. I never did complete the fist fights, races, or gwent. And the only treasure hunts I finished were the Griffin school ones. Once the main plot line is completed, you can go back and do these various side quests left over, but I doubt I'll bother.
There's gonna be some DLC content coming in the fall, will probably do that when it arrives.