We are taking our first September trip in a few days, so I can't give a report on that just yet! We've heard it will be a bit warm, but crowds should be very low. But, we have gone the week BEFORE Christmas and it was amazing. It was not super-crowded since we checked out on 12/22. But, we still got the "Disney Christmas" experience with decorations, Osbourne lights, etc. Weather was beautiful then too (warm enough during the day for shorts, hoodies and jeans in early morning and at night).
We've done Jan, Feb and March trips and weather was great then too, but park crowds were hit or miss. At those times we ran into some big groups - cheerleading and sports groups at the resorts and parks - which created a little bit of chaos. Nothing unmanageable, but something to think about.