Let's start by the mandatory summary.
Basically the hero is Yuki, an abandonned child who grew up in an orphanage, as expected he has some powers and as expected one day someone from his family shows up and tells him they're from the Giou clan and that their goal is to beat the oh so awful and evil Reiga.
First thing I need to say is that it's adapted from a BL manga so there are hints, nothing really big but they're there.
Second, I HATE Yuki, I never thought I could hate a character so much before, this guy is the most useless guy I've ever seen in my life, he's on the level of a main shoujo girl. As expected he shows up every 5 seconds since he's the main guy. I thought I'd die.
Third (very important) fact, I only started this because the cast is heavenly but I would have stopped without a second thought if it wasn't the only anime in which I could hear Kamiya Hiroshi in the Summer season.
Now trying to be a little less biased the music is awesome, really awesome, it's the only thing I really found awesome in the show, except for the villains, and the uniforms *cough*
The fights too were pretty epic and as I said it before the cast was heavenly.
Actually I don't have much more to say about this, it's obviously directed towards girls, mostly fans of fantasy settings with vampires and the likes, so if you don't fall in the category it's probably better if you don't watch.
Though if you're in the category it becomes a must watch.
I don't regret watching this, even if at times it was painful (yeah hating the main character is a serious drawback) but at the same time I probably wouldn't recommend it to anyone except notorious lovers of that kind of settings.