Like every other rubber component on the car, the strut mount bushings are poor quality, and short lived. At 130K, my car is on it's fourth set of strut mount bushings, and third set of shock inserts. After the second set of Volvo shocks failed, I replaced the shocks with Bilsteins. (I've had shocks wear out in my other cars due to normal wear and tear, but the shocks in the S80 fail catastrophically.) While I enjoy shocks that offer positive suspension control with the Bilsteins, they put more stress on the marginal strut mount bushings, and the passenger-side new bushing broke inside of three weeks after the new shocks were installed. In retrospect I was partially to blame because I apparently overstressed the car by carrying four adults and driving over a dip in the road. Miraculously this one was parts warranty, (which didn't cover the re-alignment and re-centering the steering wheel) ($160.00). Typically they seem to just make it over one year before breaking again.
The S80 continues to be the most sorry excuse for a car I've ever owned. You would be hard pressed to find a more poorly designed and built car at any price.