I was actually really happy with this. The characters are perfect - I could practically hear their voices saying the lines as I read them. The art is slightly different to the animated series, which is to be expected, but otherwise I thought it worked well.
The plot is pretty much what you would expect to happen after the events of the TV series - Aang and Zuko's mission of bringing peace between groups that have been killing each other for a century was never going to be easy. The three chapters are essentially the equivalent of maybe a season of the show, but very condensed, and with a lot less filler scenes and side-plots.
And to the comment below about Zuko's mother not being mentioned: she comes up a lot in the third chapter, and it looks like that will be resolved in the next tie-in series - The Search coming out in March 2013.
The only issue I had with this was the romance between Aang and Katara. The majority of the scenes where they interact are lovely - subtle, and yet clearly demonstrating their affection for each other. So then when they randomly start to call each other 'Sweetie' and the artist draws hearts all around the s as they stare into each other's eyes, it seems both out of character, and unnecessary.
Otherwise, very worth buying.