I consumed the first volume in less than 24 hours and the second in just a little more than that. Having recently been immersed in Donald Maas' Writing the Breakout Novel
The heroine is unswervingly strong and decisive even when what she's doing is stupid (i.e. falling in love with a vampire). She's very self aware and self-deprecating but also larger-than-life -- braver and more self-sacrificing than we could ever be. A bit sickeningly self-sacrificing but never mind that. (Maas calls forgiveness and self-sacrifice the ultimate traits for the protagonist of a breakout novel).
Also, the stakes are high -- another thing that Donald Maas harps on as being crucial for the breakout novel. Bella, the protagonist, risks her life every second that she's with the guy she loves. (Breakup Babe could relate to that, only she was just risking her self-esteem--not her life--with every Hot But Innappropriate Guy that she dated.)
It's plain old good storytelling and it's only mildly annoying that Stephenie Meyer just up and started writing this thing one day out of the blue with no years of slaving away over unpublished manuscripts, writing classes, etc - and the reason it's only mildly annoying is that she seems so damn nice that you just can't dislike her.