Seeing as not a lot of people look at the D3 A8 forum I'll PM you this info as well. A lot of the people that post the bad reviews are usually people who get one big problem and they just want to have their voice heard about the P.O.S. they're driving. My father owns one and I loved it so much I bought one as well. Both of them are 2005. Only difference between ours is he has door close assist (I don't), I have dual zone climate control in the back (he doesn't), and mine is black, his is silver.
They are amazing cars. Drive one and you will see for yourself. Most people I have talked to who have any interest in them are so scared to get involved with them because of the stories they hear but once they drive one they start to see just how magnificent the car truly is. IMO the ride is second to none. It is a very comfy ride and it has power when you need it. There are enough gadgets to keep you entertained for years to come.
The problems. Frankly we have had no major problems. I have had mine from 44,000 miles to currently around 88,000 miles and my fathers was purchased with 54,000 miles (not sure what he has now but he commutes a decent amount). The only problem I have had was I had to replace the radiator because it started to leak slightly. It happens occasionally to some D3's but it isn't a major big deal. My father had some problems with his door locks. He had to replace all four of them because the door would stop locking (I'm positive it has something to do the door closing assist so if you're looking to have absolutely no problems I would avoid cars with that option). We both replaced our brake light sensor (~$20 piece) because the ESP light would come on sometimes and it would annoy us not a big deal. Also his MMI screen broke a gear in it and it wouldn't open. There is a full metal gear kit that can be replaced for ~$60 and it can be easily repaired no problem. We have also had some problems with our trunks which seems to be the same with just about all D3's. They start to get a little bent out of shape or whatever and will sometimes fail to close/open by themselves and will need a little help opening/closing. Not too many ways to fix this but its more of a burden than a huge problem. It still doesn't leak or anything like that. Other than those tiny problems and general maintenance the cars have been a dream to own.
I have never heard of anyone having any other electrical problems. But then again there could be rare scenarios where anything can happen. Just look for a well maintained one and you should be fine.
Maintenance isn't too bad. Oil changes every 10k miles with fully synthetic oil. We started using Ams oil and have noticed a big difference in engine noise and smoothness. Tires can be slightly pricey and if you have the 19" premium wheels like we do, please avoid pot holes at all costs. You may get the occasional bubble and possible bent rim but its all about how you drive at that point My father just did the timing belt maintenance and it wasn't too bad. We got the kit from blauparts and they took care of us with instructions and special tools. I'm due to do mine as well.
Drive one and find a nice one. You honestly will not regret it.
If you have any more questions feel free to ask.