Angel Sanctuary manga
I just recently finished up the Angel Sanctuary manga by Kaori Yuki, so I figured I'd offer up some opinions on it since some people I know are intrigued to have a gander at it. The manga is I think 20 volumes all total, and as far as anime goes, the manga was started off into a three-season OVA that then just dies and goes nowhere. Better sticking with the manga.
The plot is insanely, unbelievably complicated after a while, introducing a fuck-ton of characters and smaller subplots, but the basic storyline is this: a long time ago, the Organic Angel Alexiel led a rebellion against God. Her punishment is that her soul is separated from her body, which is sealed away somewhere, and reincarnated repeatedly in humans on earth who endure lives of misery and pain. Her current incarnation is in high schooler Setsuna Mudo who, yes, is in passionate love with his sister. (Incest, anyone?) Her brother, the Inorganic Angel Rosiel/Rociel, is slowly going mad and, after a damn long time being sealed away somewhere, is obsessed solely with reclaiming his sister.
First of all, this manga is gorgeous. For sheer viewing pleasure, the art is just astonishingly, astonishingly well-done. Even if you don't read the manga or have little interest in the storyline, I highly recommend you take a look at the images. They're so finely perfectly drawn and characterized exquisitely.
That said, this is a mature, and very challenging manga. Not challenging in the sense of being difficult to understand, although if you're not paying attention it can be, but challenging in terms of the topics it touches upon. The potential morality, or lack thereof, of incest is at the very core of the series, with the relationship between Setsuna and his sister providing one of the series' 'great romances,' as well as the obsessive nature of Rosiel's relationship with his sister. There's violent rape, a ton of violence, suicide, etc. Not for the weak of heart.
If there's any difficulty with the series, it's that occasionally the characters are just all pricks. For a manga about Heaven, Hell, and everything in between, there's surprisingly little love here: everyone seems to be plotting, scheming, or out for their own ends. The love that we do see, even between the main characters, is impossibly twisted, socially taboo at best and emotionally/spiritually damaging at worst. And the commentary on God Himself, while striking (he sleeps to gather his powers, the universe is sort of his whim, and his head regent is actually taking charge of everything), can get really crushingly depressing after a while.
That said, the variety of characters is exquisite and astonishing - the angel Michael is a hotheaded asshole, Setsuna finds redemption, Rosiel is beautifully mad, and Sevothtarte is an angel so hellbent on purity he himself has gone mad. The genderbending in this thing is amazing, too - female Alexiel is reincarnated in the body of a high school boy, several characters undergo gender changes as they shift life cycles, others are so genderfucked it's hard to identify any gender, and Sevothtarte himself - Prime Minister of Heaven - has a gender secret on which an interesting section of the plot hinges.
Did I like it? Yes, but even I found the subject matter a little rough going at times. This is a fairly dark manga, although the ending is satisfying. If you feel like you can handle the topics it touches, then I strongly recommend taking a look at it but, if not, just look at the pretty pictures and leave well enough alone.