Having just finished the series of The Last Airbender and not being quite ready to delve into Legend of Korra, I went on a hunt to find material to extend my enjoyment of the old show, and The Promise seemed to be received quite positively, so I thought I'd give this a shot.
After reading, I can say it's really great! As an avid fan at this point I was keeping my eyes and fanboy-sense peeled for anything that was out of order with the canon, or things likes characters doing/saying things they wouldn't in the show, and it was pretty tight! It was co-written by the show's creators though, so that'll be why. It definitely does the job nicely of extending the series, and so far it's set up an interesting plot - one that isn't just black and white, good and evil, which is always nice to see. As long as they stay on track and stay true to the world of the show as they have so far, this is going to end up being an awesome set of books that I look forward to reading.
The only things I'm not thrilled about is that the books are being released very slowly (luckily I chose the right time to buy this one, since book 2 is coming out this month, but after that I'll have to wait with the rest of the fandom), so the story's going to be left hanging and incomplete for long stretches of time, which is unfortunate because it's going to leave a nagging feeling in the back of my mind for months (especially after the actual ending of TLA rounded the story off so nicely). That, and Amazon's price is a little high considering how short the book is (I tried to pace myself to make it last but it's hard not to finish the whole thing in 20-30 minutes, no exaggeration).
Still, lovely art that really pops, nice writing, I look forward to more! Definitely worth a buy if you miss the TLA characters like I do.