I'm guessing most are too new to have any real "running costs" aside from fuel as they'll still be under warranty, unless people are thrashing their way through tyres... nearly 2 tonnes of motor, so they can be heavy on tyres, if pushed... as far as mpg goes, I've heard anything from 25mpg to 50mpg depending how you drive it, I've never had anything more than a 20mile test drive in one, but I think most guys reckon on 40mpg+ long term... with two turbo's to spool up, it certainly has the capacity to consume derv pretty quickly if pushed hard and there's so much torque that it can be pretty frugal too.
It's supposed to be a hell of a lot leaner on fuel than mine (the supercharged petrol gets 22-32mpg most of the time, as much as 38mpg if I drive like a saint, 55mph max, etc), petrol is still a lot cheaper that diesel plus I only do 4000miles per year, probably less once I get a fun runabout, so I couldn't justify the extra outlay on a BiTDI.... plus the TFSI is infinitely more tunable, since it is a detuned S5 engine, which itself is detuned so it doesn't steal the RS4/5's thunder.... my choice was for the cheaper initial outlay, save the cash and then spend it on a small convertible for fun: it was going to be an old E46 M3 convertible, but now I'm eyeing up the new M235ci convertible... need to sort out some other big ticket items first, so the A6 will have to keep me happy for now.
In hindsight, I wish I'd saved even more, got the 245PS A6 TDI as I'd have the M3 already then
London to scotland at night (arriving early hours) including 1 hour in traffic leaving London - Got 42mpg and don't think this could be bettered other than empty car. It was a full car including luggage and 75lt tank. This is "efficiency" mode which regulates the throttle response and coasts when it can. I also almost entirely used ACC (cruise control) which most efficiently manages braking and accelerating. I got to Scotland and back in one tank.
In town it is less than 20mpg and closer to 16mpg, but I almost entirely drive in it in sports mode (more fun).
The punchline is - if you do loads of motorway miles - and i mean you commute by motorway, then its probably one of the best cars. It's aggressive as you need, and very luxurious.
If you occasionally hit motorway and want a sporty car - there are better options.