I had a '12 S5. I got rid of it several months ago. When I purchased it, I had thought it would suffice as a fun daily driver. While it was perhaps the most aesthetically pleasing car I have ever owned, it was disappointing from a daily driver standpoint. It was relatively slow, so it wasn't much "fun" for me. Also, because it was a V8, and my driving style, the gas mileage sucked. Further, I've found that I prefer/need my daily driver to have four doors, not too. So, that was a minus as well. Additionally, the niceness of the car, coupled with my obsessiveness, made it difficult for me to park comfortably in all the places I normally have to park, like downtown parking garages.
I bought mine new. There never a single issue with anything with the car. So, like, no leaks or weird noises or rattles or anything. The car was everything as advertised - it just didn't fit well with what I wanted/needed. That's all on me, not Audi.
If I had to do it all over again, then I'd probably buy an A4 or S4 before I'd buy the S5. Also, if I was going to buy another S5, I wouldn't and instead would just get an A5.