Why are they a great concept? Non-folding bikes aren't particularly heavy or unwieldy. Finding a place to park/lock your bike isn't hard in most cities (I think).
I guess I don't understand what problem a folding bike solves.
Probably in cities where crime rates are high. I've gotten 3 bikes stolen in SF already. The last 2 bikes used some heavy duty U-locks that almost cost as much as the bikes themselves. Some office buildings in SF don't let you carry a bike inside, but they don't care if you fold it up and stick it in a bag. Also, if you have a car, it's a lot easier to throw the folded bike in the trunk than taking apart the front wheel of a normal bike and squeezing it in the back.
In Amsterdam a friend of mine observed that the combined weight of bicycle and a lock strong enough to ensure that it will not be stolen is a constant.
We still haven't figured out the depth of this observation, does he mean that the lock is so heavy that the weight of the bike is negligible or does he mean that as the weight of the bike goes down the weight of the lock must go up...
I guess I don't understand what problem a folding bike solves.