Why I like them in no specific order:
1. Hmm, I've always been a sucker for fantasy - that's rampant here, but you don't have to like fantasy to enjoy these.
2. They're fast reads - they keep you interested until you're done reading them (at 5:30 the next morning because you have no self control and can't put it down. Oh wait. That's me. Nevermind.).
3. You come to love the characters. It's one of the only books I've read from a first person standpoint - it's much easier to write from 3rd person. Stephenie Meyer did a great job keeping the additional information coming so that you cared about the other characters since you were really only listening to Bella - the main character.
4. The "monsters" that are the main characters are all good hearted people. You come to care about them and realize that not all monsters are monsters.
The main characters of the books are:
Bella Swan - the main character. A 17 year old girl that falls in love with...
Edward Cullen - a perpetually 17 year old vampire that lives with his family of "Vegetarian" vampires...
Dr. Carlisle Cullen
Esme Cullen (Carlisle's wife)
Jasper Hale (Alice's Soul Mate)
and heartbreakingly there was supposed to be another book called Midnight Sun (follows exactly the same story line as Twilight, but it is told from Edward's perspective) which was unfortunately leaked to the internet. Stephenie Meyer addresses her feelings on the situation and posts the unfinished manuscript on her website if you want to really break your heart. She has it on hold indefinitely while she works on other projects.