I am a Disneyland CM and work on Space Mountain, I see what the Fastpass system really does. We distribute a percentage of tickets every 15 min. based on a number of factors but, usually turns out to be about 54 tickets, so that is around 216 passes every hour. We do let guest come back late on a fast pass that was issued on that day only (unless we went down and never came back up before park closing, which is very rare). We are trained to not stop the fastpass line, but we are also told to let standby go about every 45 seconds. We need to leave a buffer in the line so fastpass guests do not have to wait over 15 min. So you can see it is very hard to not make guest upset, especial the ones waiting right at the merge point as they re standing there watching everyone go right in front of them after they just waited 45 min.

There definitely are advantages to having fastpass but I don't think they outweigh the negatives. We get so many angry and upset guests every day because we are 1)out of fastpasses or 2)they are for so late in the day. plus all the guest who do not understand or have never heard of fastpass thinking they are being cheated out of something because they don't want to pay extra for something, not releasing they are free. We have gotten rid of fastpass in many attractions at disneyland for that fact and may be loosing more in the near future.
And for those of you who say it is not clear how fastpass works, look at a map it explains the whole system right there or, just ask any CM.
My Spiel is, "Fastpass is basically like a reservation, you take your entrance ticket and put it into the machine and it will print out a ticket for the time you come back right now between (looks at fastpass clock) 4:15 and 5:15 this afternoon and only wait about 10 to 15 minutes no matter what the standby wait time is."