Essentially you have to use Fastpass if you want to experience the same amount of attractions you would if Fastpass didn't exist.
This. And even though you're not waiting in a line for every attraction, the lines you are waiting in (for the attractions that have FP) will move more slowly. So, people should average the same amount of time spent in line, since the capacity of the attractions hasn't changed. But, the lines will move more
slowly (when I was a CM, we were told to let in 80 FP guests for every 20 standby guests), and therefore I think the overall sense of being in "long, slow-moving lines" is INCREASED. Before FastPass, the lines moves at a pleasantly quick pace, so that when queues were themed well, it rarely got boring. Now, we stand in place for a long time, move a few feet, stand in place for another long period, etc., and it makes the whole "line" experience very different and much less pleasant, I think.
That being said, some people here have stated that since fewer people are in the standby lines, FP doesn't actually make them longer (only slower, but that's counteracted by the smaller number of people in them). There are two things I can't square with that claim:
(1) At Disneyland, when an attraction comes back from refurbishment early (e.g., Matterhorn or Space Mountain), FastPass isn't available, the wait times always appear to be substantially shorter. And it's not about the season, etc., because you can see the wait times jump the very next day.
(2) FastPass also seems to make the actual line length longer, and evidenced by how they tested and removed FastPass at DL's Pirates, because it made the lines spill too far into the guest areas. (Unless that was just because they had less space for the standby queue? But it wasn't by much...)
Maybe
@lentesta has done an analysis on the effect of FastPass on standby queue length, controlling for all other variables as much as possible?