I had the chance to ride a single speed Strida with disc brakes and belt drive for a few days (only very short distance), and a ten year-old knockoff Dahon/Brompton today as well (my dad's bike - he is carfree). Twitchiness aside, I was very impressed.
Now I'm considering a performance folder primarily for use when I'm away from home (on vacation, road trips, etc), and secondarily as a spare bike for well, anything really... weekend rides, commuting, errands, whatever. I'm 5'9 and ~140lbs. I think I prefer drop bars but it seems like most people with folders end up going back to the flatbar/barend combo? Wide gearing and climbing ability is a must. Ideally the frame would be able to handle wider tires too. Don't really want anything larger than 20" wheels as that seems to defeat the purpose.
I've looked at Bike Friday's website a fair bit, and I've also looked a bit at the Reach and Birdy bike models. Essentially I would like something that rides as close to my full size bike as possible. It doesn't need to fold/unfold really fast. Budget is reasonably generous (my dad wants to buy/contribute to getting me a folding bike) but I wouldn't want to go much above $2k.
One thing I'm confused by is the suspension on some of these bikes such as the Reach. Does it a) really help the ride quality? and b) does it reduce your pedaling efficiency a lot? I have a hardtail MTB (no lockout on fork) and even on that I don't like the "bounce" when climbing hills, etc.