It also depends on the time of year. I didn't use any fastpasses this past trip - we didn't need them. Wait times were rarely over 20 minutes for the top rides, making FP unnecessary.
Here's the thing - you need to look at wait times AND the return times. Sure, you can collect them all day long, but if you're not interested in the rides with shorter lines, what's the point?
I go by the crowd calendars and use an app for wait times. Even with that information, a park can still be unexpectedly crowded - the ability to hop is very important to us. When the weather was so cold in December, a friend and I were able to do just about every big ride in the MK before it started warming up - and the crowds began arriving. About 11:30a.m., we were leaving to pick up my mom for lunch just as everyone was arriving. Lunch was in a different park. It was a 'bad' day for MK, but due to extenuating circumstances, we were able to walk on all the mountains, astro orbiter, teacups, and a few other rides. Likewise, this past trip, we were in Epcot on a low crowd day, but there were two tour groups there (unpredictable element) so we hopped over to the MK rather than battle two tour groups for the few rides in Epcot.
There is a lot that goes into making the most of FPs - on a hot day in AK, we got to Kali River Rapids the FP time was for half an hour later. The ride wait was 20 minutes. This was a no-brainer - get FPs and get in the stand-by line and we walked on after our first time down the rapids.
Another example - this past trip, the wait for TSMM was 20 minutes, the FP time was 2:00p.m. at 9:30a.m. Why get a FP for hours later when you can be riding in 20 minutes? Even with the wait time, we managed to ride it three times in a row before 10:30a.m. - way before the time we would have been riding it if we had just gotten a FP.