Just a great series. Probably the best animated television series of the 2000s. Although Spongebob during its peak was excellent too.
Anyone who's a fan of animated series should check it out. The animation and art direction is stunning, the action set pieces are spectacular, and there are a lot of laughs and a lot of heart.
@everyone: please use the spoiler style to cover up any spoilers for fans who haven't started/finished the series.