I have had a B8 for a little over 3 years now and the only problem that I have had is a sensor went out that is mounted to the intake manifold. The dealer replaced the manifold under warranty.
The B8s seem to be a little oil hungry. I usually put about 7,000 miles a year on my car so I usually just take it in for service once a year. I typically have to add at least 1/2 a quart at about the 9-10 month mark.
Aside from those things, the car is beautiful, people always notice the lights, it gets great mileage 24/32, it stops quicker than most cars, it is super stable to drive, and it is comfortable for 4 people. I really love the car and have nothing with it that I am really unhappy with.
This forum is so dead, I'd hop on Audizine if I were you. I went from a B7 A4 to a B8 S4, not exactly the same but I'm super happy with the size and look of the B8 over the older models. The new technology is definitely a plus. If you go w/ a B8 I would make it a 2010+ w/ at least Prem Plus and Sport Pkg w/ the A4.