I enjoyed reading The Hedge Knight. I was fortunate enough to read the series of books (Hedge Knight, Sworn Sword and Mystery Knight) before getting on with Dance of Dragon, and I find that this spin off series (or whatever term you would prefer) adds a lot to the main series, with a certain cross over of characters and a great deal of depth added to Westeros' history. I knew jack s**t about the Blackfyre Rebellion, and about as little about the Targaryen's before the Mad King, and this book clarifies a great deal.
Regardless, the Hedge Knight is only the first in the series, but this graphic novel did well to translate the book in my opinion. Though I don't think I can recommend a graphic novel over the books in this case, I am very happy with the purchase and will likely buy the rest of the books in the series as they come out.
TL;DR: Great visual adaptation of the book series, which is fantastic for fans of the ‘A Song of Ice and Fire’ series. It won't kill much time, but is a great addition to the experience. Recommended to read the series before reading A Dance of Dragons.