Hello.
I've tried both the Brompton and the Curve D3, and I'd like to share my thoughts, and see what everyone else thinks too.
Here in the UK, the Brompton and the Curve D3 are more equally priced -
Brompton M3L - £514
Curve D3 - £350
Curve SL - £550
We also have a 'Cycle2Work' scheme which takes about 50% off the price of bicycles. It isn't going to happen again for my employer until the spring 2008, so I've got some time to think about what I want to do.
I found problems with each bicycle.
The Brompton has poor brakes, which seems to correspond to the general opinion. That's with dual-pivot brakes (the meanest they do) front and back. The Curve D3 has V-brakes, which are much better - more what I'm used to. It rode well, though, being stable at all speeds.
The Curve D3 seemed like a good idea, on paper, with more modern components. When I tried the one in shop, it was twitchy at low speeds, and the gear change was very stiff. I don't think anything can be done about the twitchy nature (short wheelbase and aluminium frame), but should the gear change be that stiff? The Brompton's old-school gear changer was very easy to use.