If you're looking for a folding bike that feels and rides like a full size bike, I highly recommend Bike Friday!

I own two of them - a Pocket Rocket that I have set up as a skinny-tired road bike, and an Air Glide that I have set up as a touring bike. In fact, I rode the air glide across the US in 2002 - see my journal on crazyguyonabike.com. They are dynamite bikes, and the folks at Bike Friday are good people.