In case anyone is looking for A8 (D3)/pre-facelift (2004 - 2005) info, I hope this will help them shop for and choose a good used D3 A8, please add anything I forget.
We've only had our A8 for a couple months, but it's been great so far.
My advice to anyone in the market for a used A8:
1) Buy a VCDS from Ross-tech ($349) - I always wanted one, justified it because to change the rear brake pads is $1000+(!) from the dealer, and you can not do it without the vag-com/VCDS. If you get it before you go A8 shopping (as you should) be sure to expect many intermittent faults caused by bad connections and low battery*. They can usually be disregarded. It'll have FAULT REPORTED in about a hundred places and say "Implausible signal from sensor xxx" and "short" and such, but the key is to copy the report, clear the faults, and check it again after a couple days' driving - any faults that come back may warrant further investigation.
*Low battery can seriously fubar these cars to the average observer. The low voltage will set a plethora of fault codes from every sensor that relies on 12VDC (read all of them) and unless the low battery was charged correctly and/or replaced at a shop with VAG-com, they can't code the battery to the car! This fact may work in your favor from an uninformed seller whose less-than-honest shop showed them the dismal-looking fault readout from their car showing potentially tens of thousands of dollars in repairs which they are hoping to pass along to you, the car's new owner! Of course, you will simply code the battery to the car and reset the faults. We hope.
2) Read the forums! Audiworld.com, A8Parts.com (UK site), Audiforums.com audizine. All have more posters than this site,sadly. Read them all, then start over when you're done!
3) Do not plan to modify the car. It already has every-damn-thing, and the CAN-bus system does NOT take kindly to interference. This car is smarter than you. Live with it! =)
"Cons" you should consider:
1) You will insist on driving any time there is a choice because you will never be comfortable in other (lesser) cars. Which brings us to...
2) Repairs require a seperate savings account. I am seriously considering adding a savings account for "Audrey" (my kids named the car).
3) Gas mileage is 17 city and 23 at the very most highway. I had it up to 24 on one 22-mile in-town trip (never over 50 MPH) with sloooow acceleration and lots of coasting. I may install a taxi meter or a donation box in the back. And people would give, I think, because they are actually happy to ride back there!
But hey it's as safe as a tank, so oh well. It gets better than my '67 Impala (10-16 MPG) so it could be worse... ;^)
The A8 is so nice I find myself looking for reasons to go out - need some milk, honey? I'll go! Half-hour ride to go out to dinner?No prob! It will really spoil you, so be prepared to start hating other cars! I cannot overemphasize the superiority of an A8L over a standard sedan. It is better than an S-class or even a limo because it's as comfortable as either but you don't look as though you are trying to show off.
It sounds sooo great! V8 baby! (Wish it was louder).
Also, the insurance on it is slightly CHEAPER than it was on our E320, and the E320 was 8 years older than the Audi. The A8L had more rear seat foot and leg room than a 2004-2012 Escalade! Heated REAR seats! Electric and rear side-window sun shades! These are truly special cars. Yes, I am in love with a car.