When I worked for the Forest Service (many moons ago), we called them "misery whips". You need some skill, or they can be a real pain to use.

Take a look at Bike Friday for a good example of how to make a folder (http://www.bikefriday.com/). They really put a lot of thought into the engineering. I've test-ridden them and, even with small wheels, the ride is almost indistinguishable from a conventional bike.