Whereas I found book 1 and 2 tiresome at points because I'd watched s1&2 and knew exactly what was going to happen. I still enjoyed the book though because of the amount of back story you get. I'd definitely read the books before hand. GoT is a very good rendition of the books, good enough that knowing the book plots doesn't taint the viewing pleasure. Imo anyway.
But then you'd miss out on the shock tactics. I cant imagine anyone who had read the books got hit quit as hard when watching the red wedding etc as us who didnt know it was coming and im sure theres more to come.