They make a huge difference in the feel of your car, you never notice how much flex is in your chassis until you have driven it without flex.
Also another good upgrade that looks good from the side and looks good when your brake tourqing and will help you hook up and go is a good set of traction bars!.
My car has them and it the best thing I've ever done. I also went ahead added new front and rear sway bars as well. Like speedthrill says, you need to weld them in. Bolting them will not work.
you can make your own and have someone weld them in and it wouls probably be cheaper then buying a set. the bolt in ones arent solid so they aent much upgrade. with a weld the connector becomes part of the subframe and works a hundred times better
I have them in mine and like others have said, the difference was dramatic. The whole car is stiffer and handles much nicer. Also one of the main reasonson you want them is to protect you quater panels from buckling.