I've never been in September (we're usually there in early December) so I don't know what the crowds are like but I've been told that they aren't too bad. When we go at the low times in December, we've rarely used fastpasses either before or after they introduced FP+. I did select my allotment in Dec 2013 but only used one for TSMM (first time ever been able to do it!!), one for Test Track, and one for Soarin in six days. We went on all of the headliners but standby was perfectly acceptable.
We don't spend all of our time on the attractions and under no circumstances will we ever do rope drop. As far as I'm concerned, I'm on vacation and I want to relax and sleep in. We watch parades, shows, performers, eat, wander through the shops, wander through the different resorts, etc. I would say that this last trip was one of the best we've ever had. While there's always stuff that we want to do that we don't get around to, I certainly didn't feel like my days were either overscheduled or that we missed the bulk of what we wanted to do. Just finally getting to do TSMM without having to get there at RD or waiting in line for ages just to get a fastpass to come back five hours later was a winner in my view.