Audi/VW's are known for their electrical problems... Coil packs regularly going, frying ECU's, sensors failing, regular check engine lights (malfunction indicator lamp, depending on wwhat you call it), MAF (mass air flow sensors) dying, window regulators, etc...
Mechanically they are sound... Pretty stout engines and transmissions.
Audi/VW is odd about their electrical problems though... They constantly come out with revisions for parts, yet they still have the same problems. For instance they are up to revision "N" on a particular part... being that A was first, then B, C, D, etc., you get the point.
All in all not bad cars, but if something goes wrong, they are very costly to fix. Ie. Engine/Transmission problems will net you a huge bill.
On a side note I currently drive a newer Audi S4, and would gladly pick up a new S5... I happen to really like the new styling.
You may want to pick up a VAG-com (if you have a laptop, you just buy the software and connector)... this way if the check engine light pops on, you can check it out to find out it is a minor thing before taking it to an Audi dealer and paying $100-$150/hr just to diagnose a $3 problem.
Good luck...