It's a pretty cool genre mashup. There are two seasons, and I think reactions to the ending were mixed... I'm honestly not sure I ever finished it, as season 2 didn't air in America until 2013 which is well after my Toonami watching days.

It's available for streaming on HiDive, or if you have a vrv.co subscription you can watch it there. Ending aside, I really think it's worth a watch for the ambiance and atmosphere, as well as the visual style.