To reply to jbump's comment:
As long as they contain the same number of coils or MORE then the factory coil springs, it increases the suspensions vertical resistance, which can reduce body roll, and will improve handling characteristics.
Cut springs on the other hand use the same rate as stock height springs, and can cause the suspension to potentially travel too far and can cause damage if they change too much.
All performance springs follow the first style. Lowered cars that bounce excessively when they're in cities are the numbskulls who cut springs.