S5 is a great DD.
I know Kirk has a friend that's been through several winters with one and loves it.
S5 is one of the sexiest cars on the road, and if you want the V8 variant, is a "classic" sexy GT car. Add an exhaust and it's just an amazing sound and a rewarding drive.
That being said, some people do have carbon issues, a lot of which depends on how it was broken in, type of oil used, oil change interval, and most importantly, how the car is driven. The gently babied cars who were broken in softly, and get driven 2 miles back and forth to work tend to be the worst of the worst.
The cars that are given hell, but have solid 5K OCI are usually good. If the carbon buildup presents itself as a performance or driveability issue, audi will inspect and clean as necessary.
That being said, if you can live without the sound, the upcoming 3.0 S/C S5 is better at everything. Fuel economy is way up, no carbon issues to speak of, and can run low 12's with a flash & pulley. Same engine that's in the current S4, and it's a GEM.
If you want a GT car with classic V8 sound, the V8 S5 all the way. If you give the nod to performance, fuel economy, and reliability, the 2013+ 3.0 S/C S5 is the way to go.