Cost: $250
Time: 2 days, off and on
Theory: The springs lower the car about an inch, bringing it only slightly below the stock height, but they are stiffer and progressive. That means that initially, their resistance is low(er) and the car is not too harsh on normal roads. As more force is applied, more force is required to compress the spring, so they become more stiff during harsher rides, helping to reduce body roll and enhance cornering.
Opinion: Having replaced the springs, struts, wheels and tires at about the same time, I noticed a huge change in the handling. The car feels like it is glued to the road now. In a sense, it is almost scary, because the limits are so much higher than stock. Always be sure to slowly re-learn the abilities of a car after making significant changes!