"Or do you run into bad weather if you go in September?"
+1 to what Greg said. There is no better time to be in the Sierra. After Labor day the crowds thin out a lot, the weather in general is about as close to perfect as it gets in the mountains, and there is an energy in the air that is hard for me to articulate; it just has to be experienced. All that said, the weather can turn suddenly at any time of the year, so always go at least minimally prepared.