From an old school Trekie: Not a bad film by itself, a couple of minor inconsistencies, disappointing as a prequel.
What makes a prequel interesting is that you learn how your favorite characters developed, and why they do what they do. The difficulty is in keeping the facts straight. The prequel needs to adhere to the history already presented and the characters can't know about things they will learn about in the original.
Once you establish that the film is an "alternate universe" that all goes out the window, the characters may as well be completely different people. That is when the story took the wind out of the sails for me, after that it was mildly entertaining but lost the emotional attachment I had up until that point.
Given that everyone sees the books and characters differently in their own minds, and that they had to fit such a masterpiece into a two hour film, I would say that this Twilight the movie is a huge success. That being said, there were parts that set me off, but only because I am far too picky when it comes to Twilight. They ventured away from the book in quite a few parts, but overall I can't wait to see it again! ~Chi^_^