My experience with replacement springs is that they rust quick, while factory ones have that thick rubber coating. GM has tons of variations in the spring rate and static height on coils, so quick struts may be changing things from stock (could be better or worse, I suppose). The springs last virtually forever, but I did have one break once on a car, don't know why. Even the struts last like 150,000 miles or more, until the tops rust through or you dent them up somehow. I would take a high-mile stock strut and spring over a cheap new one. The upper strut mounts do need to be replaced periodically, as do the control arm bushings and sway bar end links. Usually when people say "my car needs new shocks" they actually have a blown bushing or strut mount making noise.
I do see cars regularly with a back tire hopping on the freeway because the damping is just gone. I always wonder what that feels like inside the car and why they don't seem to care. Handling has to be terrible when tires are only touching the road half the time!
