I can only say good things about it.....My 147 has powerflex all around front and rear plus a few custom made bushes that powerflex didn't offer at the time or still don't offer.
The car becomes completely different, once you drive such a car you never want to go back, as everything else feels loose.
In terms of comfort I do like it...it feels like a soccer ball going over pavement, so you don't feel the car falling apart when going over harsh bumps. Comfort wise it really depends on the tires you use!!!
I have the powerflex on my 147 for more then 100 000kms....and if you ask me if I will do it again....yes I will...and now I am going to do it on my 159 TI.
p.s: From time to time you need to lubricate the upper wishbones but that also depends on your shocks and springs if you have sports ones you will need to do it less often.
Alfa 147 QVD 300hp by BlackAngels, Alfa 159 2.4 QVD, Alfa 159 TI 1.9 QVD and Crosswagon edition Nero