I'll chime in. I was an addict of all 3 Witcher games and I read the books and comics as well. The reason why it was so addicting to me was this: your decisions very strongly shape the story and outcome of the game; the game doesn't fall victim (at least not as much as other games) to the moralizing tone of other such type of games; the game universe was so intrinsically designed; all of the sidequests, sidemissions, monsters, treasures keep you hooked onto the game; everything in the story (even the side stuff) is designed to be emotionally engaging.

The cons, in my opinion: all of the side stuff (which was designed amazingly) still distracts way too much from the main game; the hype kinda ruins it, because people go in with way too high expectations and essentially, if you don't like open world RPGs, you're not going to like Witcher 3 either no matter what; SO much conversation, on my 3rd playthrough I just started skipping through the conversations, because it really constitutes a massive chunk of the game and I already know which choices I want to go with; the game doesn't do much of a great job explaining the lore universe, which is why I went to the books. The books give a much better outlook of the psyché of certain key characters and explain the universe much clearer and better.