I've got the BC's installed on mine,
they are quite stiff, and depending how you lower, it will determine how stiff your ride is.
The nice things about these, is not only can you adjust the Spring perch on the fronts, but you can also move the entire Hub assembly mounting point/lower strut mount/whatever you want to call it. So you can either adjust the perch, or the mount. Adjusting the mount is a lot stiffer, and moves the point where your wheel sits (when you have the car jacked up, the wheel doesn't hang low, on my car the stance is the same when the wheels are off the ground as it is when they are on the ground.
With the mount moving, it allows for you to go much lower! And it doesn't affect the suspension geometry as much as adjusting the perch does.
The ride on my car is extremely rough, with the way I lowered..there's times when I hit a dip in the road doing 50+, and my car will almost become airborne, both from the dip, and the recoil from the springs.
I can't say much about the ride quality with other coils, as I had Tokico Shocks and Struts before this..but it just depends on how you lower..the mount, or the perch.
Perch - good ride.
Mount - stiff, but amazing handling.
Also, the rears don't go low enough... I've got my rears as low as they can go, and still have about 1/2-1" gap between wheel and fender..
The fronts, however, can go lower than the car could physically allow!
09 GT500, black
Kooks LT, Kooks Catted X-pipe, Borla Touring
2.6" Metco Pulley, JLT 127mm CAI
All black everything