I've got a 20 inch dahon dash P18 and it's great for the own car, for the train... And for touring in and around cities, no problem for rides until 20 km or also more. But sure, for real trips, vacations by bicycle, I would prefer a fordable big bike, like the 26 inch Dahon Matrix or Cadenza. They are still foldable, for using them in trains..., but when you ride on them you have the feeling to be on a 'normal' bike.:)
And yes, as already sayed above: easier to buy a tire!

Regards, Steffi