I'd also like to add that there is ample development in the major characters; as of now (Volume 5), pretty much none of the major characters are the same as they were at the start, and have changed that have made sense in the context of the series. And while I enjoy the fun and cute nature of the show, I really appreciate thet the writers know how to... play, with your expectations. It's one of the series greatest strengths I think.

Not to mention that the series looks gorgeous from volume 3 on.