There are a couple guys here who have the Dahon Jack. One of them is Apnu. I asked him what he thought, and he really likes it. Actually, there's nothing personal in it, so I'll just copy the text of his response:
Originally Posted by Apnu
Quite a bit. It rides comfortably with those big, bouncy slick tires. It is pretty tough, though I had to rebuild the rear wheel being a 260 lb clyde and carting 15 lbs of gear in panniers which includes : a bag for bike, clothes, work garbage and food for the day. This puts the rear wheel was under extreme stress, the manual says not to exceed 230 lbs of total weight on the bike. Since the rebuild the wheel has been rock solid. It is a simple bike too, when commuting the single twist shifter is great. Simple also means easy maintenance.
And it folds, so I have piece of mind because I bring it into my office building. I put it in a bag so the rent-a-cops don't bug out about a grimy, dirty bike on their nice clean marble floors. They seem to accept this just fine, its a good compromise.
The only detraction is portability. It does fold and that is awesome, but sometimes the 26" wheels are a bit big when negotiating doors, and revolving doors are impossible to use. Every folding rider I've ever seen (in real life or online) has a folder with wheels that are 20" or less. But 20" seems to be the standard. So I turn more than a few heads with my big black bike.
Finally, attaching extra bits to your bike can be complicated as the geometry of the machine is nothing like most bikes. This is true for all folders. So placement of stuff such as water bottles, pumps, locks, and racks require some extra though. I struggled for a long time about where to put a lock (sometimes I ride it recreationally and stop somewhere, lugging the thing in a mall is a pain, so I'll lock it then) until it dawned on me to attach the lock mount to the front fork on the side opposite to of the folding direction. Stuff like that requires extra thought, but doesn't affect my opinion of the bike at all.