The humor is great...and such a relief from the "serious" RPGs that play like funeral dirges...;) I liked Witcher 1 better than Witcher 2 precisely because it wasn't afraid to poke fun at itself occasionally. W2 got a bit boring it was so "serious"...W3 hit the balance perfectly, imo. Geralt was "serious" but also very easy-going, too.
No matter what the tone, all RPGs are complete fiction, of course. "Serious" or no...;) It's healthy to remember that...By far the biggest complaint that most people have about both computer games and technology is that it doesn't meet their "expectations." But the people who keep their expectations to a minimum and their minds open are the ones who enjoy it all a lot more. Same is true of books & movies. Don't insist that a game take you to where you think you want to go...let it take you where it will, and hang on...;)