I never would of imagined the difference it would make on my car, but it's easily one of the best upgrades for our cars. The way the car hooks powering out of turns is just awesome. It sure helps with launches too.
Be warned though, make sure the rest of your drive train is up to snuff. It will make any weaknesses in the system apparent.
My setup with a torquey 2L, built-up IRS trans with type 2 CV's is (so far) near bulletproof and a wicked fun street toy. She'll hook when you want it too, or she'll slide when you want it to.
If you get a ZF take it apart an check the clutches and the thrust washers. The clutches usually last forever, but the thrust washers can wear pretty bad.
Alex