Might as well get in before the legions of fanboys start flaming each other over which one is the "better" game.
Since Witcher 3 isn't out yet we obviously can't comment on the story of that particular one. Witcher 2 likes to pretend to be more mature, but all that really means is that they were willing to show boobs on screen before Inquisition did.
I found the story itself to be quite good though, and the choices they offered had much better consequences than what you generally get in a BioWare game. The combat is also more challenging, if you like that sort of thing.
The characters you run around with are well written, although not quite as numerous as Dragon Age since that's BioWare's main strength is their characters.
I'd agree with others that you should lookup the lore of the previous games on YouTube if you're planning on playing the 3rd. I'm sure it wont be completely necessary to enjoy the story, but you always get a bit more out of the game if you do know the lore.