I had a multitronic 140 A4 Avant for a year. 40 mpg average. Was expecting more to be honest. The 170 engine is considerably worse mind you, so if its actual fuel economy your looking for, get a BMW instead.
Also had the 177 A4 Quattro for 3 months before getting the 335d. It really was terrible on the fuel averaging only 37 mpg:
I also had a BKD engines A3 140. (You wont get a BKD in an A4/6 because its mounted differently, only for A3's and mk5 golfs etc...), and it was pretty great really. 45-48 mpg average, post map (to around 180 BHP), driven quick most of the time. I had it lowered on H&R and it was okay, if a little crashy. But did have it for 3 years / 50k, and never really had too many problems with it. Great little car, and was the reason I gave Audi another chance with the 177 after the poor fuel economy of the 140 A4. Won't be making that mistake again though.
That said, if you can live with circa 40 mpg average, they are nicer looking motors than the equivalent BMW and Merc, IMO.