I did a (ahem, metric..) century on my Birdy, and have done lots of 40-50 mile rides on my Birdy and Xootr. No folder-specific issues, though I'm still looking for the perfect saddle...
Yep folders in themselves are not a problem. What is of first importance, is how comfortable you are.
I agree 100%. There are two major components to comfort as I see it - bike fit, and road shock/buzz.
- Most high end folders can be made to fit you as well as any "normal" - especially the custom fit Bike Fridays. So getting a folder that fits you shouldn't be a problem. Test ride before buying if you can... (though most bikes can be adapted to fit).
- Suspension + small wheels = speed and comfort
My Birdy, my wife's Moulton, and the Moulton I used to own are/were all fast and very comfortable. I found my Xootr's aluminium frame to be very "buzzy". That said, I have a friend with a Bike Friday Pocket Rocket and I've never heard him complain about road buzz. Maybe it's that magical steel ride?
- Tire selection can mean a lot for comfort. I had Primo Comet Kevlar tires on my Xootr that rode like concrete donuts compared to the IRC Metros I replaced them with. Schwalbe Kojak & Big Apple tires are wide and provide a softer ride, without much/any increase in rolling resistance. However, I just replaced the stock Maxxis tires on my Birdy with Schwalbe Stelvios and am surprised that the ride comfort is just as good - even with such a skinny high pressure tire.