our "usual" time to go was always late April/first week of May, we love it, the temps can get high but the humidity is pretty low so it's not so oppressive. The first week of May crowds are not too bad at all, most rides have 20 minute waits except the headliners obviously, we fastpassed those. Wehad a few instances of riding BTMRR 2x in a row with no fp and no waits, the tea cups we rode 3x in a row, splash could be a 25 minute wait without fp....
We went in September 2013 and while we had a great time, it was verrrry humid compared to May. We found ourselves not doing much during the day but waiting til the sun went down to do most of our park touring so we weren't melting.
edited to add: September crowds are reportedly lower, on our trip in Sept 2013, we went the 18-25th and it was not low crowds IMO, not sure if free dining effected it or what but it was definitely more crowded than I thought it was going to be, seemed a little busier than our May trips. The week after Labor day though is supposed to be very low crowds though since all the kids are just back to school and etc.