The first 3 books of the series (Game of Thrones, Clash of Kings, Storm of Swords) are some of the best fantasy written in the last 50 years. Unfortunately for the last 2 books, Martin has been a victim of his own success. They aren't bad, but not to the standard of the first 3. Feast for Crows was a bastard of a manuscript pushed out too fast by his publisher. A Dance with Dragons was better (particularly the 1st half) but still suffered from Martin's excess. A lot of those issues *should* be ironed out in the show because they're doing a nice job adapting the source material.
Btw, I heard Martin is intending to fall back on prequel material if the show catches up with him too quickly, which it undoubtedly will based on his time between books.