.
Although I have the Quaife (since November 2008), I reckon that the Wavetrac is a serious contender. It's supposed to have an advantage over the Quaife. However, after thousands of miles on my Quaife (DSG version) I have absolutely no complaints and think it is an excellent mod.
Any talk about a diff being only good for track is rubbish, IMO. Your car doesn't know whether it's being driven fast on a road or a track and the diff simply responds to the conditions and how the car is being driven. There are plenty of circumstances on a road when the diff will react without you behaving as if you were racing other cars.
Most people are reluctant to spend the cost of installing a diff as a mod because it's not one you can remove and sell on later (assuming that's your inclination).
If you plan to keep your car and you enjoy spirited driving whether on road or a trackday, I recommend it without hesitation. You get used to it pretty quickly and you still need to take care when driving and stay away from the limits but it extends your car's limits. It's also a mod which contributes a lot to the overall package of a power modded FWD car.
The fact that Volkswagen have now introduced either a diff or their equivalent as OEM must tell everyone that there's a need.
Quaife Review:
http://www.mk5golfgti.co.uk/forum/index.php/topic,651.0.html 
HTH
