I had one for a while...black 2004:
I bought it with around 70-something thousand miles, sold it with 98k on it. It was okay for what it was. It wasn't fast by any means (but was faster than the 2002 Grand Marquis I had before). My old Caprice would tear it a new one, though. ...and we won't talk about how the G8 compares.
It also had the most random things go wrong with it. I had a brake caliper bolt fall out when I was on the highway. That caused the brake rotor to rotate outwards into the inside of the rim as the brakes were put on....but only in reverse. Also, the parking brake exploded inside the drum on the rear axle, causing obnoxious noise. Both of these were fairly weird to diagnose, and probably wouldn't have happened with a "normally" maintained car. Let's not get started about the condition of the paint, either. "swirls" were really non-existent compared to the number of actual scratches all over the hood.
That being said, it was awesome. Everyone moves out of your way on the highway for some reason. ...and if you go the speed limit everyone else does, too. Would I buy one again? Probably not, but it was a fun, generally reliable car for cheap money.