I recently spent a good chunk of time in Cadillac's new challenger for BMW, Mercedes, and Audi, the 2014 CTS (full report coming). But back in the day, they used to compete with this, the Seville.
The STS was a competitor for everyone, even the Acura Legend. It's 4.5 liter V8 made 155 horsepower. 155 from 4.5 liters. That's crazy.
It is smooth and quiet, but it's front drive. This is supposed to be the rival of European competitors, but it has inprecise steering and understeers.
It just isn't good enough to challenge a BMW, inside, outside, or in driving dynamics. Whoops.