I have a 20 inch Dahon. I've never ridden it for more than 2 hours at a time.....my butt starts to hurt but I'm not sure that it' the bike's fault.

It's all about the compromise, though. It's wonderfully small and maneuverable and a blast to ride, but it is a bit creakier than my full sized bike and I do occasionally wish for slightly higher gearing. Mind you that I already have a front double cog and the outer sprocket has ~58 teeth. There is only so much you can do with small wheels. No matter how fast I'm pedaling, there is always some giraffe on a full sized bike leaving me in the dust.

If I had to ride a bike all day continuusly and needed folding, I would probably reach for my Montague full sized bike.