This is one of the best animation shows ever produced in the US. When it first aired, I was with my kids who were four and seven years old. We were actually waiting for a new episode of Spongebob which would usually air on Friday nights. When it was Avatar: The Last Airbender, we were initially disappointed. I had seen the previews when watching Nickelodeon with my kids but it didn't appear too interesting to me.
We watched anyway and they aired the first two episodes and my kids and I were hooked. In the following weeks as we continued to watch more episodes, I started to tell all my friends to watch it. I lost count how many people I got hooked onto Avatar. It doesn't matter how old you are, the characters are great, the writing is fantastic and it's just an overall great series. I own all of Hayao Miyazaki's movies, including a few before his started Studio Ghibli. While, the animation is not on par with his movies, for a TV animation series, the animation is great and it's obvious they were influenced by Studio Ghibli, which is not a bad thing.
If you haven't seen Avatar: The Last Airbender, do yourself a favor and watch it. You will be hooked.