I have a Formula S18 and use it a fair amount for commuting on the train here in the Boston area. Much of the time, I don't bother folding it because the double decker commuter rail trains have bike racks.

The only flex I find is in the stem but if you keep it tight it is not a problem. I keep a narrow profile 8mm box wrench with me for a quick tighten every now and then. This bike is such a pleasure to ride.