When I was a child? I liked to run them fast. Now I run conventional sans cruise control on O-27 track, and none of my locomotives cost more than $150, including shipping, though most I have upgraded with ERR RailSounds. Let's say the most I have invested into a single engine is about $280, still very modest. While I can't run my trains as slow as those of you with cruise control, I still generally try to run them as slow as possible. So I think that some of it is related to youth
Every once and awhile, the kid in me sneaks out and I open up the throttle. My series-wired Williams geeps, heavy as they are, stay planted. The lighter K-Line MP15, also series-wired, becomes hairy as it approaches the full-throttle, and would almost certainly leave the track if pushed that far. The Lionel docksider, heavy for it's size can run quite quick, but I have managed to dump it on my O-27 curves. My layout has plexiglass fences on the sides that aren't up against the wall, so the risk of damage in a derailment. But again, usually my trains don't leave the slow to medium spectrum.
Interestingly, when I go to the local club's semi-annual open house, it bothers me not when children run the trains on the hands-on layout at high speeds. However, there is one adult who likes to run his scale passenger train on the big layout at above-scale speeds. Even though it is his prerogative, I find it mildly irritating.