I plowed through all four books in eight days. That might indicate an obsession, a speed reader, an easy read, or some combination thereof. In the case of Twilight, it's a combination.
I'll say up-front that I recommend these books, at Amazon's amazing price, for anyone interested in Vampires-meets-90210. It reminded me of a coming-of-age, Dawson's creek-style story with supernatural elements.
The books are very easy to read. The first-person narrative, written like a journal or diary, gets you into the character's head and keeps you turning the pages. Though the perspective changes between characters later on; I found that switching to be a weakness. You can't bare a character's mind and soul, then suddenly switch to a new mind and soul and ignore the previous one--the readers are cheated out of a deeper insight.
The plot is interesting enough to want to finish the books; however, it does kind of eye-rolling at certain points. There are definitely a few teen drama moments and more than a few repetitive statements. Those episodes of juvenile insecurity and monotonous dwelling weaken the story; thankfully, it's not often enough to ruin the entire series.
Mrs. Meyer gives us an amazing glimpse into the main character's life, and the author can write some pretty enticing--but not too explicit--sensual scenes. Vampires and sensuality go hand-in-hand, and the author ties them together well.
Lastly, the overall quality of writing is mediocre. While I'm not a professional, I do understand the basics of grammar and voice enough to know that Twilight, and its successors, displayed plenty of weakness in mechanics. There is a lot of adverb and adjective overkill; the proverbial "Show; Don't tell" was abandoned in many, many scenes. The use of tag lines (she hissed, he chuckled, she replied, he retorted) is abused throughout the books. When in doubt, use "said" to tell us who is talking.
Grab the books for the price; don't expect the next Harry Potter or Anne Rice-caliber novel, but take the story as is and you'll enjoy it.