There may be possible spoilers in this so if you haven't seen the movie and are planning to you may not want to continue. I'm going to try not to give too much away though ;)
Overall... I thought they did a really great job. It was done as a low budget film and you can tell. When I read the books I saw Bella and Edward as happier people then they are portrayed in the movie. Also.... they left out some of my favorite scenes... the blood typing scene in the biology room and the scene after they meet James and Bella is formulating their plan of escape and Emmit decides he really likes her b/c she's "diabolical". LOL! That's just a few. They shortened her hiding out time w/ Alice and Jasper by... a lot. You didn't get to know them as well as you do in the book --- in fact, you don't get to know a lot of them. It does show Edward playing Bella's Lullaby but if you didn't read the books and don't know that's what he's doing then you'd never know. They never say "I love you." The day that Bella is asked out by three different guys is barely touched on and really only is asked by Mike... Eric starts too but is interupted and Tyler never even tries so of course Edward doesn't get to act all sneaky and stop him. hehe
Stephenie Meyers plays an "extra" in a cafe scene. Oh! that's something else. Bella doesn't cook for Charlie in the movie, they eat at a diner all the time. The guy who plays Charlie is awesome -- I loved him. Alice is JUST like a pictured her. Just like a pixie! There are several direct quotes in the movie but they did have to combine a lot and the movie felt rushed (similar to how a lot of the Harry Potter movies went). As I said earlier.... I totally think it was worth seeing and I recommend it to any Twi-hards out there. They have to make 2 million dollars in order for Summit to approve making New Moon so go see it and if you have the money then go see it again!!