IJ is right, due to the geometry, there is a slightly progressive effect to the suspension, but this is not as much a progression as progressive springs.
I don't like progressive springs. With a good damper, there is no need for them. Like others have said, they're soft under small displacements and get stiffer as the spring compresses. They will make the ride more comfortable, but matching a damper to them is hard, so the damper will only be effective during part of the suspension travel. Usually it will have to be soft, so it doesn't overdamp the lower displacement part of the spring, so then it's too soft for the high displacement rates of the spring.
Basically, they're good for street cars and people who aren't really going to drive their car. I wouldn't suggest them for any type of autox/track days/drifting/etc. People will say they run them and that they do alright, and if they think that, then imagine how much better it could be with a good spring. They also won't provide as good feedback as a good linear spring which is very important when you're trying to go fast.
Tim