I've been following this series for quite a few years now and I can honestly say that it is one of my all time favorite series, fo' sho'.
I just love the characters, the way they interact, and how close their relationships to one another become. More so since at the beginning Teito really isn't the most sociable person, but later you realize that the relationships within this series are kind of a big deal, especially Frau and Teito's (but that may just be because I'm biased). It's also nice to see how well the characters develop throughout the series and seeing how much Teito, and the people he affects, grow.
The story also develops and unfolds rather nicely, there are some slow points every once in a while, but it's all important to the storyline more or less, so don't jump the gun and drop the series just because of something so trivial.
Lastly, the art is just beautiful is does justice to the story. It may seem "girly", as most of my male friends have called it, but they all enjoyed it once they read it. So don't let it scare you! And I also will point out that it's not a Shounen-ai series, though I will be more than happy if it became that way (I can sense that sort of connection happening with Frau and Teito at any moment now).