By the time I got to three friends having told me I really needed to watch this series, I started to take them seriously. The biggest problem was trying to watch it on the library's very popular and very scratched DVDs: I was forced to skip "Book 2 Volume 1" (how they label the discs, in this case indicating episodes 1-5 of the second year out of 21 episodes) because the disc was unusable. Each volume has a run-time of 2h02m, made somewhat shorter by five sets of beginning and end credits. The series ran for three years.
I found several problems with the series, none of them particularly serious. It's heavy-handed with the life lessons for the entire three year run - often with the young characters delivering them with more understanding of emotions (and willingness to talk about them) than 12-to-15-year-olds actually have. But then, the target audience was (according to Wikipedia) 6 to 11 years old. The animation is relatively low rent, low res, and jerky - although it's also often quite beautiful. Several pieces of the outcome were blatantly obvious: the Avatar would succeed, and I knew where Zuko and Iro were headed by the middle of the first season (although Zuko's journey took longer than I expected). But despite these short-comings, it was immensely enjoyable: it's funny, it's great to look at, it's amazingly inventive, and it's really charming. I was a little shaky on it after "Book I Volume I" (the first five episodes) because it was TV animation and heavy-handed, but I kept going and was richly rewarded: they take the time to create a huge cast of characters, and to tell their stories. Some parts are predictable, but most of it is surprising and very entertaining. It's a crying shame that Shyamalan's live-action movie interpretation is so widely reported to be spectacularly awful.