Do you ride a folding bike?

If yes, what are the good/bad aspects? Which retailler would you recommend?

Yes, a Giant Halfway that I got in April of this year. I like the fact that I can get it in the backseat of my car without using a rack. It comes with carrying bag, a rack, bell, and fenders. The Halfway 20 inch wheels, which give a slightly more bumpy ride than larger wheels, but I replaced the stock seat and that helped a lot. Other folding companies offer 16 to 27 inch wheels, depending on the models. The Halfway is actually is pretty speedy. I can pick it up and carry it to hard to reach spots. It has seven speeds, which is fairly decent and shifts well. The folding mechanisms work well and it folds up or down in seconds without any problems. The cables have a slight tendency to get caught in the folding handlebars, but is not a major issue; I just have to watch them. It adjusts to most any size rider. It is very easy to get on and off of, though different models of folders will vary on that. The Halfway is also attractive and definitely gets double looks from people. ("What kind of bike is that?")

I would go with a local bike shop that has a Giant, Trek, or Dahon folding bike. They will help you determine what you want and give you maintanence on the bike.
I've got a Brompton – lovely little thing. Folds up like origami so I can take it on the bus to work.

They're not as comfortable as a full-sized machine and I've never seen one with decent gearing.