I bought this game on a whim. I hadn't even heard about this series, now I'm glad I did.
The single-player story is pretty fun, and there are a lot of one-liners and inside jokes laying around. The best parts are those that are outside the main objectives; the secret, hidden underbelly of Paradise.
The one thing I like most about this game is that there are essentially no rules. You can choose to complete an objective the obvious way (say, by waiting in line to get Gary Coleman's autograph,) or the fun way (by killing Gary Coleman and taking the autograph.) I like not being penalized for killing the wrong people.
I would definitely encourage anyone who buys this game to also download the "Eternal Damnation" mod, which can be had for free at places like Filefront, etc. This mod is in reality a whole new game with new weapons, a new story, a new hero, new voices, and re-purposed locales from the original. And zombies. Lots of zombies.