The story's nothing that will win awards, the 'taking back the streets' theme is a little tired. The humor's a little crude, but it's good enough to earn a cheap laugh or two. But it's damn fun to mess around in.
The best way to describe it is as a huge cartoon sandbox. You can pick trash, rocks and people up and use them as weapons. Clothing and cars are customizable to a surprising degree, down to the pattern on your tie or whether your shirt is tucked in or not. You're rewarded for everything - driving on the wrong side of the road, kicking people while they're down, flinging yourself from your car in a crash - everything. The rewards make just driving around the city satisfying, since you're never without something to do. No matter where you turn, there's a diversion, as the game calls them. And I've had more fun with these than I have with the actual missions.
The FUZZ diversion, where you impersonate a police officer for a COPS-like show, is exactly how the Vigilante missions in every GTA should have been. You're given random crimes to deal with as brutally as possible in order to get higher ratings for the show. The crimes range from riots to streaking to carjacking. The best part is the police join in on your side, and assist as you violate the civil liberties of criminals everywhere. The police attacking you in the GTA Vigilante missions always ruined the fun for me, even if that's a tad more 'realistic'. The variety of crimes, police siding with you, and generally higher level of mayhem make it a lot more entertaining.
The only problem I have with the FUZZ diversion is that the time limits are a little harsh, and I'm rather have more time to mess around, but you can restart with little to no hassle.
The performance issue is the only thing holding the game back for me - the stuttering while driving is terrible, although part of that might be my computer specs, a lot of it seems to be poor optimization for the PC.