SPOILERISH if you haven't seen any Avatar episode.
AVATAR: The Last Air Bender took me by surprise. Tuned in mostly to satiate my Miyazaki fix. Season 1 (or Book One: Water) was great: Jaw-dropping action, great animation and very, very funny. Its finale was quite reminiscent of Princess Mononoke in theme and execution and I was slightly turned off. Slightly as in nanoseconds of slight Hey, that's familiar! but which immediately passed.
Book Two: Earth was Avatar's Empire Strikes Back! or Aliens 2 or LOST's Ana Lucia season. More benders, more Fire nation assasins, Zuko goes from evil to good to evil. Appa gets lost and goes wild. And Aang falls. Falls in love and falls defeated and dying at the end of the series.
More than any cartoons out there, and second to my must watch TV shows this season under LOST, tied with DOCTOR WHO and TORCHWOOD, and above HEROES, AVATAR has got me seriously hooked. And it looks like Book 3: Fire is bigger and better and has been confirmed by its creators as the last season (of this particular arc which means what exactly? No more Aang?).
There will be no sleeping.