the only 2 brands that i have any experience with are tunit and the psi tuning box. these are definetly not cheap resistor type tuning boxes, these are not to be confused with the cheap resistor boxes off ebay.
as said before it worked fine on my golf gt tdi 130, i noticed a slight drop in MPG but i was driving it harder, but i have read of people having them fitted for thousands of miles and having no problems. as with a remap your putting more power through the clutch and more torque through the gearbox, but these vag tdi's seem to cope with it all really well.