SEA_Tigger just beat me to the punch! I just finished a rental with the 500.
Overall, I was favorably impressed. As noted, the acceleration was much better than I remember in the Taurus. My vehicle had leather seats and a Moonroof, which came in handy on the traffic choked Edens expressway. Lots of legroom for my 5'9" frame. Fake wood grain accents are nice.
Powerful air conditioning unit. The stereo system was nice, although I had poor reception on some stations I normally have no trouble with. Not sure if that's a vehicle flaw or not. The display is a bit dark for my taste as I had trouble seeing it in the daytime.
But, compared to a Taurus, this is a step up. When renting F class (or lower), if I see a 500 in my stall, I'll be pleased. Not as pleased as I would be with a Camry or G35, but pleased nonetheless.