This collection leans heavily toward cute, fluffy anecdotes about the Gaang at play between episodes. Of those, I think my favorite is "No Benders Allowed," both for giving one of my favorite secondary characters a cameo and for the awesome hats. Even the stories that show the Gaang training or fighting are generally pretty light--not that that's a bad thing. "Private Wang Fire" in particular had me snorting out my nose. Other stories add tasty trivia tidbits, like the fact that Zuko once studied with Master Piandao, or show us characters that make only offscreen appearances in the show itself, such as Kuzon.
For most readers, though, the strongest reason to pick up this collection is the set of stories that falls between seasons 2 and 3: "It's Only Natural," "Going Home Again," and "The Bridge." All fill the critical gap between the fall of Ba Sing Se and Aang's awakening and help flesh out some of the character motivations in early season 3. And if you're going to read those, you might as well read the rest of the collection for the added amusement value.
One other story deserves a shout-out for being tonally quite different from the rest of the collection: "Relics." Its final lines pack almost as much punch as the close to the episode "The Blue Spirit," and its art is stand-out in a generally (though not always) well-illustrated collection.