First off, I'm not a teenager and I haven't read the book. I watched the movie with the rest of my family, because my teenaged daughter insisted that we should, and assured us that we'd all love it. (In return for this, she has to watch Dracula ...)
Actually, I watched it twice, because it was both much better and much worse than I expected. It's clearly a teenage-girl wish-fulfilment fantasy, starring teenage actors, so you don't expect anything very thought-provoking, any serious characterization or (probably) any Oscar-winning performances. But I could be wrong about that.
The acting is first-rate, in particular Rob Pattinson's portrayal of the dark, brooding outcast who "doesn't want to be a monster". The barely-controlled anger, the pathos and the vulnerability he brought to Edward were quite outstanding. I'd also give five stars to Cam Gigandet's James, the Tracker, whose combination of a feline predator with a sadistic sicko was also superb. The developed relationship between Edward and Bella is everything that teenaged first-love should be -- desperately intense, sweet, touching, and sometimes laden with embarrassment. They were great. I thought Kristen Stewart's Bella was OK -- but the character is a wimp (the kind that just wants to be protected and loved and cherished for ever but doesn't seem to do a great deal to deserve it) so she didn't have much to work with.
On the down side, the "willing suspension of disbelief" takes a real beating with this movie. Some of it is rather obvious and my daughter tells me you just have to live with it -- like the fact that Edward has been 17 for "a while" but still doesn't act any older than this. (I mean, he's been going to high school now for *ninety years*? Doesn't he get bored with it? Doesn't he fall asleep during the ninetieth repetition of Grade 11 math? Since they obviously aren't there to meet other kids, why don't they pretend to homeschool? How come no-one notices that they never eat their cafeteria food? How come they still behave like teenagers after ninety years? Because if they didn't, there wouldn't be any story, of course.) Bella appears to go from outright dislike to undying love in about two days, which made me want to shake her and tell her to grow up. (You've known him how long??) The real let-down, though, is the special effects. The first time Edward raced up a hill at superhuman speed, three of us (everyone except my daughter) just fell apart laughing, it was so patently, absurdly, just a speeded-up film. It looks like Charlie Chaplin. Leaping from a balcony, or across an enormous room, was unbelievably fake. (Real jumps have a trajectory; only a zip-wire doesn't.) With the best will in the world, it's hard to immerse yourself in an adventure that suddenly springs these awful gaffes on you.
On the whole, though, I quite enjoyed this. Of course, 99% of teenage girls adore it, but their parents and siblings can watch it too without being overly shocked or appalled. Just try not to laugh or comment too loudly, because it annoys the teenage daughter.