That was a good move on your dealer's part. An overnight test drive gives you a chance to pick up on things you would have missed on a quick drive.
That said, from MY perspective, the Stinger is a great car with only a few shortcomings. Personally I cringe everytime I see a bump ahead because I know it's going to send an uncomfortable jolt through the whole car. Then there's the HK system, which is just decent; I'm always glad I didn't pay extra for it. The climate control is maddening; there's supposedly a way to disable the auto-ventilation but it hasn't worked for me--every few minutes it switches from recirc to fresh air, even though I DON'T want it.
These are just a few of my small nitpicks. Overall it's a great car and I'd buy it again.
Good luck!