I've driven a gen2 haldex equipped car, it does have a switch next to the ESP button so you can put it back to stock. In my opinion it isn't worth the money unless you do track days. Money would be better spent on a remap.
Go have a look at davefish's blog on the r32oc for his views on the haldex, and then see JayB's report on his customcode.. you'll see the light

I'm trying to get an evoms induction kit over from the states at the moment, will let you know how that goes.