nomis wrote:Thanks for the feedback so far, please keep it coming!
Well, I've done 31k miles in my RS 6, and I've driven 3.7, 4.2 and 4.0D A8s as well as my fair share of old A8s/S8s, so hopefully I'm not talking total dribble...
The RS 6 is a fantastic car, for what it is. It inevitably gets compared to pure-bred sports cars because of it's performance potential, which I think is sometimes a little unfair. quattro GmbH were basically given a cart horse (nobody can call the C5 a real driver's car with a straight face) and told to make it run the Grand National. I think given what they had to start with, and the resources they had available, they didn't do a bad job.
However, it's a C5. The engine and gearbox are heavy. They are a long way forward, and a lot of weight sits over the front axle. The DRC suspension trickery makes for nice flat high-speed motoring, and limits roll on corners, but it can't defy physics and take out the understeer that's inherent in the design. Really the RS 6 is more at home on reasonably fast (80-120mph) sweeping corners, but above that it can become a bit detached and floaty, and in much tighter corners, understeer will chew away at the tyres and usually one of the electronic nannies steps in and really hampers your progress - it's programmed to act later, but it's just as ham-fisted when it does as the rest of the Audi range.
All is not doom & gloom though. The surge of acceleration is addictive, as is the noise it makes as it does so. The gearbox is quick-changing mechanically, even though DSP sometimes lets it snooze in Drive. Either use Sport mode, or (my preference) keep it in Tip out of town and make your own gearchange descisions. The Bose stereo is reasonable for a factory-fit, there's better out there, but there's a lot worse as well. It's pretty bassy, if slightly 'muddy' sometimes. With 8 pistons, the clamping power of the front calipers is impressive and makes for rapid stops, but the pads are small and the cooling isn't sufficient to withstand repeated heavy use, and it's easy to get the front pads smoking with a fairly short spirited drive.
I still love it though. That shove as the engine thumps and thuds (in a good way) it's way through the revs is incredible, and barely lets up through the gears. It will keep pulling and pulling. It can be "twanged" round fairly fast corners (80-90mph) and it isn't unsettled by repeated left/right transitions under those kind of conditions - good for english minor A roads and better quality B roads.
So, to the A8. Well, it's got far more toys and a much more modern feel. Having driven a few variants, I'm quite impressed. I got to attend a pre-launch day at Millbrook proving grounds, where I drove one round the Alpine track, high-speed bowl and through a slalom & 'elk test' set. Even with the suspension set to Comfort, it can be thrown around by even an inexperienced driver with a lot of tyre squeal, but no real drama. The chassis is quite well sorted for such a big car and the dynamics are surprising.
The A8s I've driven all pretty much handled the same way regardless of engine. I hope this means the design is reasonably well able to be balanced with differing engine weights, so hopefully the W12 will still handle as well as it's V8 siblings. I'm supposedly booked in for a test drive in May, so I should know more then. If it's as good as the V8-engined A8s, but with most of the RS 6's performance, I could be a convert.
One thing bothers me though - the brochures show LWB and SWB versions, but Audi UK's web site only lists prices for LWB. Audi UK tend to view the prist lists as authoritative, if it isn't on there you don't get it. I hope either they do bring an SWB W12 over, or the LWB doesn't affect handling in a bad way if they don't.
Not a fan of BMW, Audi preference for sure!!! 
