A couple years ago, we bought my wife and I a couple of folders.
I wanted internal hub gears, (thought they'd be a lot less trouble with alignment and cleaner) and liked the idea of the 16" wheels, so I opted for the Curve D3.
Kate wanted something comfortable and didn't care what kind of gears it had. I didn't think the single speed would cut it with our local hills, so I got her a Speed D7.
I wish I had also gotten a D7. The D3 doesn't have a granny gear, so climbing hills is tough. Standing up and pumping results in the handlebar twisting in its stalk. Not dangerously, but annoying. A low gear is really necessary for a folder for this reason. I like the idea of the puncture-resistant Schwalbe Big Apple tires, but either because they're fatter or because they're smaller diameter, it is more effort to ride this bike than the D7. Also, the folded size between the 20" D7 and the 16" D3 is negligible. Lastly, the D7 was about $100 less.
The D3 has needed shifter adjustment a couple times. This morning, it wouldn't stay in 1st gear; kept trying to jump to 2nd.
The D3's wheels are not quite even. They're a little thicker at the seam, so that when I brake, it is not smooth at all. The seam has torn a bit of rubber off the pads, so now the screech as I'm coming to a stop.
The D7 has cheaper tires. (Kenda) We haven't gotten punctures on either, but the D3 has a lot more miles on it than the D7.
What would make it better is slightly lower gearing all-around; smaller front sprocket would do it. That, or a 2-speed coaster brake shifter, like some Schwinn bikes had in the 70s. That would eliminate the cables going to the rear of the bike and make the whole thing more trouble-free. The first gear would be very low, the 2nd gear would be low/medium. With these small wheels, going fast is not a good idea.
The stock seat sucks, and although they really tried with the grips, they are not comfortable. They would be better if they were just plain round ones, without anything sticking into my palms.
I have had more seat time in my D3 than her D7, but the D7 is a nicer ride.
Overall, I give the D3 a 6/10. Build quality should be better for the price. (referring mostly to the non-uniform wheels) I looked into sending the wheels back, but they wanted the whole bike shipped back, on my dime.
I give the Speed D7 a 8/10. Smoother ride, more stable, faster, less expensive, more gear options, and the fold is only a tiny bit bigger than that of the D3.