I'd like to start by saying that I have not read all of the Twilight series, I have only read part of the first one. Also, I don't want to offend any Twilight fans, this is just my opinion of what I read and you are more than entitled to your own.
Quite frankly, I hated it and couldn't bring myself to finish it, which is rare for me, usualy I try to finish a book once I've started it. I found the writing style infantile and the characters were either bland or hateful. Bella is dull and seems to be that way so the reader Twilight is aimed at, teenage girls, can impose themselves over her quite easily. Edward was unrealistic and annoyingly 'perfect' to the point where he wasn't really a character at all but a cardboard cutout. Both lead characters are also incredibly selfish in their actions, putting not only themselves but the others around them at risk just so they can be together.
Also, vampires do not sparkle in the sunlight, end of.