Going 170 km/h on a straith highway and feeling the rear "move" as a passenger isn't exactly reassuring. And I didn't trust the driver!
My dad owns a Mercedes C 220 cdi, he never said anything about the rear braking out.
On a track it might be fun, but in normal traffic? ::)
Thats a fault,be it mechanical failure on the rear suspension of some sort, or, more likely a fault in the drivers ability. make no mistake.

No BMW 5 Series be it an e12, e28 , e34 e39 nor an e60 display such tendancies, believe me, I've driven most models of the 5.
As for traction & rear wheel drive, its all down to balance of the car and throttle position. For what its worth, I drive with my "traction control" switched off 95% of the time.
Posting drivel here since Jan 31st, 2002. - That long!
"He who laughs last, thinks slowest."