Yes, of course if you've read the books you should definitely go see the movie. And yes, you will probably enjoy it. BUT, that said, as a book-to-movie adaptation, of course it could also be better.
Personally, I always wonder where they find the screenwriters who're crazy enough to take on a job like this. No matter what you do, no matter how faithful you are to the book, or conversely how much you change it, very, very rarely is a film version ever better than the novel that stemmed it. Twilight is no exception.
Certainly, it has it's good points. The casting, for one, I think is exceptional: while I had some qualms before I saw it, I loved all the actors, and wouldn't change a single one. They have their moments, of course, but for a young cast they're really rather impressive, particularly Robert Pattinson: I'm disappointed Catherine Hardwicke wouldn't let him play the part as 'dark' as he wanted to.
And that, I think, is where most of the problems with this film lie: with Hardwicke, its director, and Melisssa Rosenberg, it's screenwriter. For starters, I can't help but question Rosenberg's credentials. She's written a lot for TV, but only one other screenplay to date (that being 'Step Up', of all things) and sure, everyone's got to start somewhere... but with one of the biggest films of the year? Maybe not.
I can't fathom many of the changes Rosenberg made. Adding in scenes like the extra scenes with the 'bad' vampires, thus shifting the films emphasis from the 'love story' to the 'action' or 'horror' factor, or like the entirely pointless Biology trip (the conversation that took place there could easily have happened in its original setting), and shifting the order of scenes... the entire Port Angeles scene was thrown out of sync by the fact that Bella did not find out Edward was a vampire until after. And then when she planned to confront him, she went off into the forest with him, not knowing he was 'vegetarian'! In the book, I never thought of Bella as crazy... in the film, I can't help but question it. I simply do not understand the need to change things like that, which would have been so much better if left as they originally were.
Then there's the fact that both Rosenberg and Hardwicke seem greatly preoccupied with showing of the 'scenery' of the the American Northwest. Great, so there are lots of trees. We don't need to see Bella and Edward running off into the woods and climbing trees (besides the point that Edward would never let Bella do that anyway because he'd be too scared of Bella falling out of one), because its supposed to be from Bella's perspective... and Bella doesn't like the great out doors and Forks, in her opinion is far too 'green'. So, skip the theatrics, and give us the love story.
But, err... what love story? If you have not read the books, you could call Bella and Edward's romance entire non-sensical. They've barely talked to each other, and suddenly they're saying 'I'm only afraid of losing you' or 'you're my life now'. Although, never once do they say 'I love you'. And isn't that what it's really supposed to be about? The ultimate love story. Sadly, it's not.
But,all that said, i will see the film again, and I will be buying it on DVD because the essence of Stephenie Meyers work is still there. The plot is still good, the characters still similiar (though Bella is more annoying), and Kristen Stewart and Rob Pattinson do have good chemistry, they're just not given enough chance to prove it.
I really hope they do better with examining the love story in 'New Moon'... but seeing as there's less of it, I really doubt it.