My experience is not that heavy of usage, but with buying them as new old stock. The grease that Lionel (or whoever makes them over there) puts on the gear and shaft hardens up which puts the whole works in a bind to the point that the truck will derail because it starts rotating since the gear can't.
So if you get one and it starts derailing frequently, take the mechanism apart, clean out the old grease, and reapply your favorite flavor of lube.
I typically run one or two of them during Christmas-time shows at our modular club. Everyone seems to like them. I think I have an elf chasing a reindeer and Mrs. Claus chasing Santa. They typically run for about an hour a day, and are as trouble free as anything else in the train.
Hope this helps,
J White