It sucks in many ways, not just because I don't like it for no reason but because of some legitimate concerns:

1. Bad grammar/editing (I don't know how they let these issues slip, but it did happen. Although the errors border more on the line of personal preference, there are some examples I know of that were not added for literary effect.)
2. The character development is either non-existent or just downright horrible. Bella never makes any significant changes. She is essentially the same person from beginning to end. It's a matter of the things she experiences and the places she goes to that change. Edward is very much the same in most ways. Really, her secondary characters have more personality than the main ones do.
3. The plot is horrifically boring and predictable. There is nothing exciting or different about the Twilight series. It only offers what we've seen more times than I can count in a slightly different manner, and with some weird and often ridiculous twists to the story.

All in all, it's not a piece of literature that should be praised or commended in any way, shape or form. Unfortunately, it is because it appeals to the cheesy romantic, immature and sexually charged audience who crave badly written series like that.