I'm currently playing in a weekly Buffy game. The system is ... decent. Not my favorite by a long shot. Personally, I think the Drama Points have too big of a place in the game's mechanics - they're really the only way that normal humans can fight against supernatural baddies, at least in my experience. They don't feel appropriately dramatic if you can expect to spend one every turn you're in combat, as well as on every important roll through the course of a session. It's possible our GM is just giving us too many - it's a new system for him as well.
The game does a fair job of capturing the feel of the show. I like the magic system, with the division between ritual magic and real-time sorcery. Combat flows pretty well, and the system penalizes repeated actions, which makes things more interesting than 'Attack, damage, attack, damage, repeat ad nauseum'.
I would personally rather use d20 Modern for the game, but the other players wanted to try the system out. There was talk of using GURPS, but I really don't think that would be an appropriate fit - BtVS is too cinematic for GURPS, as far as I'm concerned. I think d20 Modern would fit well because it's a good, solid system, with Buffy-type games as one of its primary intentions. It's properly cinematic, without removing all element of challenge. I'd use the rules from the BtVS game for the magic system, and perhaps find some kind of drawback system, but other than that, it'd fit well.
The books themselves are excellent - they do a great job of capturing the feel of the show, with quotes and pictures spread liberally throughout. Very high production values there, similar to the (relatively) recent Star Trek core rulebooks.
The PCs in my campaign are a Watcher (me - an English teacher with a background in ecological terrorism), a nerd-type, two jocks (one a Potential Slayer, the other just a normal human), and an amateur demon hunter. The campaign is set in the setting from the Slayer's Handbook, Grizzly Peak. It's a fairly amusing setting, with some good NPCs and such. Of course, since I don't really know how much of that is my DM and how much is the book itself, that's not much of a helpful bit of advice.
Anyway, I'm rambling. It's a decent game, though not precisely to my tastes.