Petrol or diesel? If it's a 2.0TDi engine there are quite a few issues with the oil pumps failing leading to expensive rebuilds. I had a 2008 A4 Avant S-Line 2.0TDi that had covered 54k miles for four months earlier this year. I didn't get on with it, so quickly sold it at quite a loss. The engine felt lazy and hesitant with very little power (140bhp model). I have a 2012 Mk6 Golf 2.0Tdi which is a much better engine.
This link provides some useful info. The oil pump issues appears to affect the B7 2.0TDi engines and even early B8s. Apparently the 1.9TDi is the one to have.
http://www.audi-sport.net/xf/threads/2-0-tdi-oil-pump-balance-shaft-failure.81997/As mentioned the rear windscreen washer pipe gets blocked, mine was sorted by Audi, but went again in under a month. The shock springs are also a weak point and snap. The interior was great, but everything else about the car was a real let down.
The other thing that frustrated me with the car was a really bad condensation problem. I changed the pollen filter and checked all the drains but even with the climate control on, it was still pouring off the windscreen.
I won't go near Audi again for a while.
