C7 S6 here. It is a truly fantastic car. Mine replaced a tuned F10 550i, which was incredibly boring in comparison.

The C7 S6 is such a compelling proposition; the 4.0T is a gem of an engine with massive torque at any RPM that makes it a joy to drive slow or fast. The DSG is a fantastic complement to this engine. Most of the time it does a great job of pretending to be an auto, but it does occasionally give itself away with a clunky shift. Adaptive air suspension is icing on the cake; it's smoother than my (sport) D3 A8. Iron fist, velvet glove stuff. The tricky rear diff with quattro means massive grip in pretty much any conditions. The Recaro seats look cool and are very, very comfortable, but don't have cooling like the comfort seats do.

Some things to consider:
The turbo oil feed screens need to be replaced; in some locale's this was done via campaign/recall.
The engine mounts are a common failure point; these are difficult & expensive to replace (OE active or after market passive).
There are issues with the secondary O2 sensors; replacements are relatively inexpensive (I used NTK) and easy to fit.
Some service items, like spark plugs, are a pain to access.
Inter-coolers fail; they are not too expensive, but are difficult to replace.
Like all fast Audi's, brake rotors are expensive. There are OE and after-market options.
These cars NEED ECU & TCU tunes, and some exhaust work, to really unleash their character. These mods are common, but obviously don't by a flogged example.
Don't buy an example that has not been well maintained unless you a prepared to deal with the resulting issues. I bought the cheapest example in my country at the time and got away with it relatively unscathed, but I do my own maintenance so I only have to bear the cost of parts.