I will continue to update this review as I ride the bike for a longer period of time, but am starting today as this was the first extended ride (~15 miles) on the bike.
I had a 2008 Jack, which was a dream to ride, soaked up every pothole NYC could throw at it - it was like riding a Hummer, you just glided over the road. Those big apple 2-inchers were just amazing - but slow as molasses. I also wanted more than 7 gears, so I looked at an upgrade - and found it in the 2009/2010 Dahon Vitesse P18 (according to Dahon corporate, both years model is the same).
It has 20" kojak tires, which after almost 3 years of a 26" folder on an Abrams-tanklike frame, felt a bit wobbly at first, and you can certainly feel every pebble you ride over. BUT, they are quite nimble, and offer a degree of road control and maneuverability that a big wheeled bike could only dream of. Squeezing between cars waiting for the light to change is much easier now, and the V18 is just flat out faster than the Jack. I still will get passed by roadies pumping hard, but they will have to work harder to get by me, if I am pedaling with effort.
It looks like most of my time will be spend in gears 14-18, as the lower gears are too light unless going up a steep hill. The bike is a feather compared to the Jack, and requires less effort to move with velocity. As expected, the smaller wheel base does require more pedal rotations to move a set distance than the Jack.
It is a more fun bike to ride in general, and after an initial extended ride - the maiden voyage of the starship Vitesse - I can say so far I am pleased. I was going to upgrade the pedals and brake pads immediately, but looks like I'll only need to swap out the OEMs pads for Kool-stops as the pedals are serviceable.
I will try to update this review every few weeks as I become more acquainted with the bike, and experience additional positive/negative events. In some ways, I miss the simplicity of riding the Jack, where there was no need to ever look down at the road to view the condition of the road. But I think (hope) the tradeoff to a more sophisticated, capable and faster bike will be worth it in the long run.