xXx is not a film that tries to innovate the genre of action films, nor does it pretend to be very clever. It's just in it to be mindless action with witty one-liners, and there is nothing wrong with this. The creators know this too, which is good, because there's nothing worse than watching a film that is convinced of its own (non-existent) intelligence and (imaginary) intellectual depths.
The story of xXx, therefore, is simple. A group of terrorists intend to do bad things to the world, the government is trying to stop them. But due to an uncanny ability of the bad guys to spot and eliminate government officials, the head honchos decide to try something new, and they send in a criminal instead. Now, since this is the hero of the whole thing, and we're talking Hollywood, this 'criminal' is more of a 'loveable bad boy with an anti-authoritarian streak.' After all, we wouldn't want the audience to hate our protagonist, would we?
Said anti-hero, Xander Cage, X to his friends, is played by Vin "My biceps are thicker than my head" Diesel. He, more than anyone else, seems to grasp just how ridiculous the basic premise of the entire film is, which is why he wears a "I can't even begin to try and take this seriously"-expression most of the time. And surprisingly, this works better than convincing acting would have, lending a realistic touch to an otherwise unrealistic whole, giving it a "B-movie gone A"-feel.
So basically, if you enjoy explosions, witticisims, and the occasional girl in underwear, try xXx. But if you're looking for something a tad more serious, I recommend you look on.
One side note, though. What the hell happened to Samuel L. Jackson to make him act this badly? I am normally an utter fanboy of the man, but seriously, what?