I went to this movie with three friends, only one of whom had read the books. I didn't have to fill the other two in on much of anything. The plot is simple enough that transferring it to a movie wasn't hard, and they only really skipped out on the history of the Cullins. No explanation on how they got together, or about Alice's lack of memory... but other than that, I can't think of much that was missing. Of course, as in all book-to-movie transfers, bits and pieces were moved around and shortened and changed a bit, but... I really, honestly, cannot complain about one thing.
Especially how beautiful Robert Pattinson is. :)
Long story short, I was blown away at how well the transition went from book to movie.
For someone who has never read the comic book, I was able to understand a lot more than I was expecting to. I personally prefer longer movies, so the 2 hour mark was no problem for me. All together, great acting, and a great story. I'd see it again in a heartbeat.