I have a B5 Avant with the full drievtrain powerflex'd.
Yes they squeak, but that is down to poor fitment as much as hard poly.
Jury is out on long term use on ARB's, the problem is they remove all torsional movement from the bush which means the Arb rotates inside the bush and any road grime acts as an abrasive destroying the ARB in teh contact zone.
they sharpen the handling up very well, little bit more road noise.
Well worth it