The first season, for the most part, was good. The second season was not so good, until the very end.
I really hated how they mangled Nuriko's character in the second season. She/he was a really complex character (essentially a woman in a man's body) and then they make some lame excuse for it.
I've only seen the first OAV, but that was kinda lame too.
It had such potential, and the first season was so good for the most part, and then it seemed like they just got tired of it.