Depending on the car it can make it over-steer even worse and on other cars it can inhibit the ability of the car to turn-in properly.
Since they aren't reversible I've kind of stayed away. I added it to the Porsche 911 Turbo '81 and it seemed to make the snap over-steer way worse while at the same time making it really hard to turn-in on sharp, low-speed corners. Ended up buying another 911 Turbo I hated the kit so much. Probably great on certain cars while being horrible on others.
Wish I had better answers for you. It'll probably be a while before we really know when it's good to use and when it's not given the economy is so tight that it's hard to buy cars, let alone duplicates to run wide-body kits for testing purposes.