Although it would not be my first (or second, or third) choice for a week at Disney, we were able to enjoy ourselves last week. We went to the parks early, stayed until noon or one, then returned to the same park later in the day. We rode without fast passes in the morning (saving them), and used them when we returned in the evening. We were never turned away even if the "window" return time had passed. Overall, although it was crowded, we had a great time. We did bail from the MK on New Year's eve around 9 pm....it was just too nuts.
One thing that kind of surprised me was the lack of understanding of the Fast Pass system. I mean, they virtually beat you to death with the info and people still can't grasp it. I was behind a very upset woman at the RnR FP machine that had just gotten fast passes for Tower of Terror, and was trying to get them for RnR 5 minutes later. When the CM explained that she would not be able to get passes for two hours, she went into a rant.
On a positive note, we had a VERY magical thing happen to us at MGM on New Year's day. We arrived at 9 am and rode Tower of Terror. Before getting on the ride, my youngest got fast passes for a second ride. Well, I neglected to get the park tickets back from her, and while we were in line for rock n roller coaster, I asked for the tickets back. They were gone from her jacket pocket. These were tickets purchased as part of the disney soccer showcase, so there were no names on them, and whoever found them could have used them, as they were not "finger scanned". Luckily, an honest person found them and turned them in to Lost and Found! It was the one time that I did not write the serial numbers down from the back of the tickets, so we would have been out of luck!