I've had a folding bike for a long time, and while I liked the old one as a bike, I didn't like the way it folded. It was messy, the chain would fall off, it was floppy when folded, and was difficult to lift and manipulate too.
So, I bought a new one. This one is by the same company (Bike Friday), but is a much better fold. It's the "Tikit" model. It folds incredibly easily (they say 5 seconds; for me it takes around 10 as I haven't practiced enough yet), and you can wheel it around on one wheel when folded, you don't have to carry it. It also stays together when folded if you do need to pick it up.
Here's me with it unfolded:
Here it is in the process of being folded:
... and here it is, as one friend put it, as a pile of spaghetti. Expensive spaghetti: folders don't come cheap. The rounded part of the frame on the right acts as a handle to wheel it around.
I really like it. It's very comfortable, and I actually use it as a folder now that I have a bike that will fold well and not flop all over the place when folded. I keep it in the bottom of my hall closet. I haven't taken it on the subway yet, but plan to. (Bikes are allowed on the subway but still, will be much easier with a folded bike, I think.)