Twilight is tolerable. The setup is quite good, but wasted (vampire politics are glossed over in favor of Bella's heartbeat). If you want to see what can be done with it go read Alicorn's Luminosity (discussion thread) which starts out a decent pamphlet for luminosity, then drops it and turns into a good book. (As in, not by the standards of online fanfic. By the standards of books.)

Twilight itself is just your usual crappy romance novel. It finds a few emotional buttons (danger, lust, beauty, resisting attraction, the high of infatuation) and presses them like a monkey on crack. It's quite possible to enjoy that. Secondary characters, while rather flat, are appealing - for example Carlisle Cullen's virtues can be genuinely moving.

I say grab it if you have time to waste, if the first five minutes bore you drop it, otherwise finish it.