XP2 can handle a whopping seven stops of overexposure (not recommending that, though!) but is not at all good with underexposure. I've heard some say that they've pushed XP2 with good results, but personally, if I want to push film, I'll push Tri-X.
Lots of people go for the high-contrast look of XP2 at 200 or 320. At 400 it tends to look kind of like Delta 400 without the grain. Portrait photographers tend to blast the hell out of this film to blow out unwanted skin texture and make eyes and eyelashes pop. I don't much care for that look most of the time, but it can be quite eye-catching.