I had those Eibach springs
LOL. They rode very smoothly. More so than OEM, but were lower and stiffer. They are great for those wanting a more sporty look, slightly less body roll, but do not want to compromise ride quality. I only had them for about a month before selling them, and bought them brand new from the Tire Rack.
I think the progressive winding on the springs is shallower so that the initial spring rate is softer than stock. You feel like you the road is smoother. However the spring rate increases progressively to become stiffer than stock by about 10% or so (this is my guess) when you are maneuvering the car more aggressively.
I personally switched because I wanted no, or very little body roll, and was not happy until I ended up with the 550/700 rate that I have now (like 5 times stiffer than stock). I am sure most people would not be happy with what I have for daily driving on bad roads, but I am building an autocross race car.
So overall: these springs that the seller has are a very nice compliment to the Z4 for those looking for a moderate increase in ride quality, appearance, and overall handling.