I have a 2005 A8L
I regret ever buying it. Although the car drives nice and handles well, it has been plagued with problems from the start.
The biggest thing, water leaked in from the windshield - to the air filter and down the steering column. This fried 3 computers which are incredibly right under the driver's side footwell (don't they know that there is a potential of water pooling from your shoes?) The total repair came to over $10,000. This happened in the first 6 months of having the car and Audi was adamant about not covering this under warranty (said this is an environmental problem). It's a design problem and they should have covered it.
Had other glitches along the way and at the 51,000 mile mark (just right after the warranty), the MMI Screen got stuck in the up position for some reason and the whole MMI system is not working anymore. I don't want to find out how much to fix this but they are not getting another dime from me until these issues are fixed .
Not worth to sell the car and take the huge depreciation - so I will mumble and groan about this car until I get another one to replace it.