Okay. I bought my folder from Jay Gaerlan in San Francisco (www.gaerlan.com). He buys basic folding frames and builds up the bikes with custumized components. Mine’s a Dahon Helios frame with much better wheels and brakes, 24 gears instead of the normal 7, and front shocks. It’s a little bit heavier than the regular Helios (due to the added gears and shocks), but still weighs about 25 lbs. I think i paid about $700 for it.
While it certainly isn’t slow, it’s not a bike for speed. It’s also not a bike for long distances - the most i’ve ridden on it is about 30 miles. However, it can’t be beat if you’re combining cycling with other forms of transportation. Just this past weekend, i took mine on the train from SF to San Jose for a dopefest, then caught a ride back with someone and stuck it in the back of his car - can’t do THAT with a full sized bike!
A few friends of mine ride Bromptons and really enjoy them. They’re a little bit more expensive than mine was, and have smaller wheels - 16" vs. 20". I’ve ridden around on them a little bit, and while they’re very stable, they’re much slower and heavier than my bike. The big advantage of them is that they fold down really small. Another friend of mine rides a Bike Friday. Riding that is damned near identical to being on a nice full sized one, and it can fold down small enough to fit on a carry-on bag. Only problem is that it’s about $2000. Heh.
I’ve heard some good things about S&S Couplings (http://www.sandsmachine.com/), but i don’t have any personal experience with them myself.
One disadvantage to folding bikes is that they tend to be tougher to carry than full-sized bikes. Because of the frame shape, you can’t just throw it over your shoulder like you can with a diamond frame. The added weight doesn’t help here either.
Hope this helps!