My family run a whole load of Audi's.. I'm the black sheep having a Merc!
We have: '97 A8 4.2 Quattro Tiptronic 25,000m - this is the same engine as in the V8, but with more power. It's not as torquey low-down as some Mercs, but makes up in mid and top end, with a lovely growling V8. Ride not as good as MBs, but it's silver and makes you feel like you're in the film 'Ronin'. Quattro makes it very desirable.
'94 A6 2.5 TDI with 235,000m - no problems except leaks a little water, only had it 8 months, but full Audi dealer service history. Only ever had service items, not even new suspension, rides like 80,000m
'87 80 Quattro 1.8 injection 120,000m - doesn't quite compare with a 190E, but nice enough for an '80s car, fundamentally reliable.
'97 A4 1.9 TDi (110 bhp) 105,000m - excellent, reliable, but not the quality and desirability of a Mercedes.
All are fundamentally reliable, though occasional problems can occur (as with any car), it's never anything big (problem areas: central locking, electric windows - make sure you check these). The family runs an independant garage, and we wouldn't have Audis if they weren't reliable!
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black