Oh my God I love these books. I kind of adore angel mythology anyway, and these books mix in 'actual' angels (as in physical beings with huge, functional wings and amazing voices who sing prayers to the heavens to intercede on behalf of 'mortals') with fantastic world-building, different cultures that interact and have prejudices and traditions and all the things real cultures have, and characters who have to make difficult choices and have torn loyalties.
It's a very shades-of-gray type world. There are angels who are villians and good-hearted characters where you least expect them. (There's one group of people who are almost always portrayed sympathetically, a nomadic culture who seem modeled after Native Americans, and another culture who are almost always portrayed negatively. In fact is these books were ever made into movies (which would make me explode with joy, srsly), this race would probably have to be PC-fied, because they've always reminded me of the more radical Muslims... women aren't seen by men who aren't relatives, they wear burka/headscarf-inspired clothing, and honor killings are a fairly prominent feature.
Did I mention that these books are considered fantasy-romance? Except they're really neither. Despite what you might expect from the covers, there's no magic or supernatural occurances. There are science fiction elements and even a touch of steampunk in certain books. And while the relationships are the focus of the stories, it's not fluffy romance novel-romance, it's crazy-angst will-they-make-it romance.
So, I really love these books.
That is all. :D