The boss fights on the Witcher 2 aren't it's strong point... while they're "cinematic" and I don't mind the occasional quick time events, most of them are pretty cheap (and can be stupidly easy if you stock up on bombs beforehand). The difficulty level jerks around a little bit... while I'd say it generally gets easier after the prologue there are times when you're walking through everything wondering where the challenge is to suddenly getting absolutely stomped in seconds. There's a few balancing issues as well, Quen in particular seems very overpowered at this stage.
I've only fought one boss, and the frustration during that boss was mostly due to me having trouble figuring out just how to do it. Especially the second phase was a pain, because I actually did it right the first time, but failed, which convinced me I was doing it wrong.
The stealth bits of the game are even worse. Thankfully, it seems you can knock out guards without penalty, so it's only hard if you decide you're going to, for whatever reason, do it without knocking out the guards. Doing that is easy enough.
Combat as a whole is difficult, and targeting is slightly wonky. My biggest complaint there is that if you attack something, you can't hit anything else, even if it steps in the way of your sword. Locking on enemies during a one-on-one fight is a good idea, but if you're fighting a group, it just gets in the way.
That aside, it is an exceptional game. I don't know what it is about the setting that makes me love it so, but dammit if it isn't better than 95% of all fantasy out there. I just like how it's not based on the typical medieval western Europe, but rather central Europe. Also, the author seems to have a thing for redheads. And sex. Add a touch of prejudice and racism ( something I find they've done very convincingly in this game ), a ton of darkness, and frequent role reversals, and you've got yourself a riveting story.