Technically the goal of Fast Pass has never been to reduce wait times. The goal was that people would view their waits differently, because of where they were allowed to endure them. PERCEPTION.
For example, you walk up to Space Mountain and see the wait is 60 minutes long. You see that the Fast Pass return time is 90 minutes in the future. If you get a Fast Pass and return 90 minutes later technically you are waiting longer to ride that attraction, if the stand by wait time was correct.
In that 90 minutes you decide to go experience the Carousel of Progress. You probably didn't wait that long at all to do that and you see that the wait for Buzz Lightyear is 20 minutes, so you do that twice. You grab something to eat and then you notice that your Fastpass return time has arrived. You skip back to the line where you wait 20 minutes at the most. I don't think I've waited that long with a Fast Pass very often though. Technically you've waited almost 2 hours to ride the ride, but your perception of the wait is that you weren't waiting at all.
If you had waited in that 60 minute line, maybe you had an ice cream with you for the first 10 minutes, but for the last 50 minutes you're just staring at the people and objects around you. This can be fun if you're waiting in one of the more interesting queues, but some of the others are less appealing.
Most of the FastPass lines I've been through have little or no wait. If there is any noticeable wait, then it's because ALL lines standby, fastpass, and single rider are backed up because of some technical problems with the ride.
If a ride has a Standby wait time of less than 20 minutes, maybe 30, then I'll probably opt to wait in that line. Even then, especially if the FP return time is really soon, sometimes I get a FastPass anyways and then ride twice in a row. The same waiting time for two times on the ride.
If it's more than 30 minutes, then my preference is to let Disney play mind games with me a little and spend my wait time elsewhere.
Also, if you have a Dining Reservation planned, getting a Fastpass before that ADR can allow you to spend your wait time in an air-conditioned restaurant, and return to your ride after you've eaten.