I've had a 2007 Vitesse D5 for about 8 months now, which has been my first foray into the world of folders. I like it a lot, though I'm still not sure it's the correct compromise for me. It sits somewhere between being a little too big and heavy to be easy to take on public transport and a bit too minimal to be really comfortable on less than smooth roads. At the moment I'm feeling I'd probably be in a better position with 2 bikes, something like the downtube FS for longer rides at the weekend, and the Dahon Curve SL for short distance rides involving public transport.
I've replaced the stock pedals for MKS quick release ones, saving 300 grams or so, and I'm currently using a really minimal lightweight saddle (again the stock one is unnecessarily heavy). I've removed the front mudguard, seat post pump, and rear rack and the bike is now almost light enough to be comfortable lugging around stations. I wish the bike would roll more easily when folded but it requires some effort to keep it balanced. Getting through ticket barriers isn't too much trouble, but I wish I could shave another kilo off the weight and get closer to about 9 kg. Instead I'm going to increase the weight a little by adding a suspension seatpost (totally overpriced like all Dahon goods) as road surfaces in London are pretty bad.
I'll look into Schwalbe big apple tires too and softer grips to try to reduce judder.
So yeah, nice bike, great accelleration on smooth roads, very nippy. But without suspension I find I'm having to concentrate hard to avoid any craters in the road which the bike really doesn't cope with well, and I'd rather be able relax more when riding.