I've done three September trips.  You can check out the trip reports here   (You need to Login or Register and get an idea of what I encountered.  In sum - as has been noted - expect everything from freezing to hot, from clear skies to rain and wind.  But you don't have to worry about finding campsites because the place is much less populated (generally speaking).  In any case, no matter when you go, you should be at your campsite by 2:00 pm - in the summer because there's often competition, and in the spring and fall because you don't have as much time to set up.