We have done a fair amount of riding, but not on folding bikes. I rode one of my father-in-law's, and it was a fairly well built bike. The change in position and dynamics bothered me. That may be because I'm accustomed to road bikes, and the wheel size of the folding bike is much smaller. That being said, a friend rode one on a 250 mile ride that I went on, and he had no mechanical problems with it at all. His comment was that it wasn't made for that kind of riding!
As far as holding up, my in-laws used for several years (light use). I don't remember what brand they were. I would guess there are some good ones, and some not so good ones, just like any other style of bike. I hope someone else will chime in with better knowledge of this kind of bike than I have.