Enough has been written about how good this series is. True, it takes a while to get going in the first season, where there are more standalone episodes, but it really takes off in the second and third. I don't watch many animated series, and for the record some of my other favourite shows include stuff like The Wire, The Sopranos, Breaking Bad, Game of Thrones, Deadwood and Battlestar Galactica. Don't let the fact it's a 'kids cartoon' deter you from the frequently great storytelling, animation, and characters. The spin off 'The Legend of Korra' is a pretty great follow-up so far too, with even more impressive (and HD) animation.
A note on the packaging. Previously each season was released in fold-out digipack packaging. A 'Complete Collection' was released with those cases inside a thin box, with a lid at the bottom, which post people found very cumbersome. This new release from September 2013 has each season in an individual DVD-sized case housed inside a slipcase with the same artwork. It's much neater, easier to access the discs and, at time of writing, is half the price of the previous release.
If it's of any interest, the release appears to be disc-for-disc identical to previous UK releases, containing the same special features (or lack thereof) and 4:3 aspect ratio. Each of the individual seasons also name the show by its UK title 'Avatar: The Legend of Aang' on the cover/spines, whilst the outer case calls by it's original name 'Avatar: The Last Airbender'.