Spoilers ahead, folks.
I'll admit, when I first heard about this, I wasn't too keen on it after hearing about the incestuous themes present in it. Little changed when I read the first few chapters, and I thought about dropping it. However, due to a few intriguing bits and pieces here and there, I was convinced not to.
And I don't regret it. A few chapters after Setsuna and Sara run off to live happily, an Angel working for Rosiel decides to kill Setsuna to get back in Rosiels good books. However, just as she is about to, Sara blocks off the...cosmic-beam-thingy, and dies. Setsuna then "awakens"-but he goes crazy and destroys the Earth......wait, what?-right? Well, fortunately, Adam Kadamon turns back time, then halts it. He then instructs Setsuna on how to get to Hades to retrieve Saras soul. Thus begins the REAL "real" plot of Angel Sanctuary. Which for some reason they decided to forget about in the anime.
What follows after is nothing short of a fantastic adventure filled with beautiful artwork. I found that the characters get more and more Sympathetic along the story. They all have interesting backstories (the main ones, in any case) and characters. I found the scenes in Heaven particularly of interest. Sevothtarte was an intriguing character to me. The focus is not all on Setsuna either, so we see a large cast of different and interesting characters. The plot just gets more and more complex, better thought and and more surprising towards the end.
I was quite surprised that Sevi was Lailah. And Sara was Gabriel. etc, etc.
Well, I've been summarizing the plot for a while now, so, in a review, I can say that if you can stay true to the manga through the first few chapters, you will be rewarded by amazing artwork, characters, and dialogue with a fantastic plot.