I have a D7. Seven gears is 'better' for me. The spread is such on the D7 that 7th is good for 21-22 mph, which is quite 'thrilling' on a 20" wheel bike, 1st is low enough to take the sting out of mild hilly terrain with shopping or luggage or headwinds. I haven't ridden the others. The D7 (I've had both the Speed and the alloy Vitesse) is a sturdy evolved bike, with improvements arriving every year. The Dahons come with a five year frame warranty, extendable online.

10 miles each way on the flat is about 30-40 minutes of riding. I easily manage 14-18mph and I have wonky knees. The Dahon saddle is good for about an hour. :-). The Speed and Vitesse are a nimble, fast and comfortable ride, and the frames are stiff and responsive.

The Boardwalk is Dahon's earlier tubular frame, it's used on many other bikes by different manufacturers, and is Dahon's lower priced model. The Speed and Vitesse model use Dahon's Sonus tubing, (a triangular section) which is said to be stronger. It's somewhat better looking, I think, and a 2009 Speed may be better value than 2010 Boardwalk. Dahon's paint finish is very good, the frames are primed, colour-finished and then clear coated, other than on the matte-finish models. Bikes come with touch-up paint, a user and service manuals, and a stupid strap trouser clip which falls off. Carry Bags are availble for them 'El Bolso' and slip cases for public transport. The bikes come with an integrated purpose designed kick-stand.

The rest of the Speed D7 is mostly traditional bicycle, cup and cone bearings etc and all of the componentry and finish is very good. The rear gear mech is a Neos, designed for the bike, and very smooth in operation. Spares are cheap, and widely available online or from a Dahon dealer. The only 'must' with any of the bikes is to get the spoke tensions checked after about 10-20 hours riding, as wheels bed-in. Gear and brake cables will also need a tweak at this point.

Lighter is better with folders. Sub 12Kg is preferable, for me. A rear rack is a big help, as are bottle rack mounts and fender mounts on the frame. The Speed has all of those. If you keep the bike in good order, Dahons keep their value, and used ones sell easily.

And yes, I'm a Speed/Vitesse fan. Black ones are faster.