The stealth missions are frustrating, but the shooter missions are at least passable.
The worst part of the game is that you spend a tremendous amount of time wandering around trying to figure out what to do next. The game just isn't very interesting, so there's no incentive to sit there for hours trying to solve these things. The "map" system is almost useless for finding your way around or locating objectives; it's only useful for keeping track of nearby enemies. Overall, the game doesn't distinguish itself in any way, shape or form. Even in below average games, there are usually a few things that are cool or fun or at least out of the ordinary, but there's just nothing like that here. A few sections are moderately enjoyable, that's about it.