I agree. Wheelsize does not single-handedly decide the ride. However, I think there is a reason why bike makers emphasize features such as size and fold when it comes to 16" bikes: 16" bikes are primarily marketed as urban commuters. (the typical urban commute being a ride to the subway/bus station - folding it up - riding the subway/bus - unfolding it - riding again.)
If people ARE riding 15 to 20 miles comfortably each day (on 16" wheels), I look up to them. Seriously. They are obviously seasoned riders who don't really need to be advised on any kind of bike-purchasing decision.
After reading all the great things about the Brompton in this thread, I think I'll give it a try myself.