I have a couple of folding bikes which I use for commuting. I rarely fold mine though. It was just a person with a folding bike at a tradeshow in chicago that rekindled my childhood interest in bikes so I got a couple of folders.

Folders can make very nice bikes. They ride much like big framed bikes except that smaller wheels turn faster. Some people consider that twitchy while other consider it responsive. Gearing on most folding bikes is lower and narrower than most full sized bikes although you can get folders that have a wider gear range if you want and need the gears. And a final parting thought. Lighter is way better when talking folders that you have to fold and carry. One of my folders is about 19 pounds and the other is about 30. The 19 pounder is SO MUCH easier to carry around when folded.

And do checkout the folding bike forum here.