Definitely darker, sexier, and more graphic than Stephenie Meyer's relatively innocuous Twilight series, there's a whole lot here to capture the imagination of older teen readers.
I couldn't help but get echoes of Harry Potter -- Zoey Redbird is "marked" for a life she didn't know she was destined to have.... she's sent to a special school for others who have also been marked where they can learn about their history and powers... she has to stumble through all the new-school jitters any young person would have to ("Where's my locker? Which elective is best?" )... and most especially in that Zoey is a young woman with a strong moral center even in the midst of lots of clearly questionable behavior (in both mortal and immortal worlds).
Some may object to the blatant Wiccan overtones, but in the overall context of a really pretty entertaining read, it's just part of the setting, and I certainly felt no pull to convert.
I'd hesitate to put this in the hands of anyone younger than about 15 or 16, primarily because of some sexually graphic passages, and a whole lot of discussion about them. That's my own generation speaking, though, and for those of us who fall well out of the "young adult" category who enjoyed Meyer's books, this series looks to be another good diversion.