And Casillas hasn't been that good for the past 3 years. It was not like Victor was letting everything in and Casillas was saving everything before Valdes became better. We usually tend to exaggerate the past achievements of other players. Take the Brazillian Ronaldo as an example, I have watched almost every game he's played except when he was in Real Madrid, I was a fan and I know what he was capable of, but now you see people saying Messi is not as good as him and some are sincere. I remember him squandering chances like Villa all the time, losing possession very easily, his passing wasn't on Messi's level and of course Messi's dribbling is far better thin his, but people only remember the greatness and overtime his skills are magnified in our heads. Same goes for Victor Valdes, we remember him for his mistakes at a time Casillas was considered better, but that doesn't mean Valdes was crap. He was, at times, an excellent keeper.
I don't claim to be an expert nor do I claim to have watched every game with those guys in them, but I believe Victor is on Iker's level, if not better. After all, he's been the better keeper (at least statistically) for the second half of the decade.