Had a 2006 B7 A4 2.0t, car was a mechanical disaster. Pretty much drove around with spare coil packs in anticipation of them blowing at any time (had one go every 5-7k from about 36k onto when I just got completely frustrated with the nonsense and got rid of it at 100k). It was easily the most unreliable car I've ever owned. That car was truly awful from a reliability standpoint, I had to get towed off the Jersey turnpike on 4 separate occasions (and I don't live in Jersey!)
I now have an S5 and, while nowhere near as bad as the 2.0t, it's still the second most problematic car I've owned. I don't think I'll be shopping for another Audi in the near future, though sadly I like them more than comparable BMWs or Mercs right now so you never know.
F06 650i Carbon Black / Black
Audi S7 Phantom Black Pearl / Lunar Silver
E93 335i Sapphire Black / Coral Red