I came across a pretty good article stating why all Avatar Fans should read the The Promise.
Here is a tid bit: 'The Promise' is an Avatar: The Last Airbender comic series that has grown rather popular since its debut. The comic will feature three total parts which intertwine to create another adventure that Avatar Aang and the gang must endure after defeating Fire Lord Ozai.
Some fans may believe that this comic is not worth reading, mainly because it's in a comic-format, not a television broadcast. But they have to realize that reading this comic is just as important as watching the show.
This comic acts as a bridge between The Last Airbender and The Legend of Korra, and if you don't read it, you won't understand the full story line of the show down the road. The Avatar Series is so dynamic and layered that its creators, Mike and Bryan, couldn't resolve all the issues on television. That's why by reading 'The Promise', you get the true Avatar Experience that Mike and Bryan have wanted their fans to get all along. Read the rest here.