Epic car, epic costs. I've spent £6,500 on mine since I bought it in October and there's still more to come, I'm sure. I'm currently averaging 18mpg on Super Unleaded. You'll find these cars change hands a lot when someone buys one, panics when the first big bill comes in and sells it on.
High points (for me)... power, traction, interior, build quality/feel, noise, 'sleeper' appeal. Low points... you won't pull with this car, plus the running costs as above.
Things to look out for if you're buying one...
Gearbox - excessive torque convertor slip, harsh gear changes. £6k+ to replace.
DRC - the car should be perfectly level and should corner flat. If it rolls excessively or dips down at one corner, you've probably got a leak. If it's riding too high that suggests the system has been recharged and over pressurised.
Electronics - check nothing has got damp. It can be very expensive to repair.
You're going to find a lot of C5s have had the temperamental DRC replaced with coilovers - it's what I did. It does transform the car and absolves you of any potential DRC related worries in the future. Lots will have the Milltek exhaust too - either resonated or non-resonated (the latter being the louder variant).
The honeymoon phase is well and truly over for me and I'm beginning to see the car as a massive extravagance. That said, I smile every time I drive it. It's faster than pretty much anything I come up against, it makes every journey effortless, it will carry 5 people, a dog and luggage in comfort and it's still respectable enough to take clients to lunch in. It's the perfect car.
Edit: M5/M3 vs RS6? I had the same dilemma and just decided that the big Audi made the performance a lot more accessible. You need quite literally no skill to drive an RS6 quickly. Try it in an M3 and you'll end up in a hedge.
Edit 2: A pic of 'orc', courtesy of Phate