(I haven't read the entire manga yet, I'm only up to the end of Vol. 5)

This has to be my favorite series. Ever. The art is beautiful. It flows really well, and it's just pleasing to look at.
The story is really what drew me in. The characters have just enough back story for you to connect with them, but there are also things that you kind of have to figure out for yourself. The characters are all so tragic, but it's not out of control. It's believable tragedy.
The characters are really likable (I even kind of sympathized with the antagonists at certain points...) All the characters are at least a little likable.
I prefer the manga to the anime myself. The anime left out a bit of the story, so I would rather read the manga (but don't get me wrong, the anime is just as amazing)