We've had three A6 4.2s in our family and I was the person primarily responsible for all of them. I still oversee one of them which is now owned by a family friend. They are NOT trouble free cars, but if you get a good one, they're very nice.
Problem areas: emissions related items in fuel delivery and exhaust systems; coolant leaks; oil cooler leaks; gear-shift/ignition key interlock problems (can't remove ignition key); airbag warning light sensor; rapid tire wear (even with proper alignment, just the nature of the beast); side windows going inop when partially open (there is a retrofit/fix for this); deteriorating leather steering wheel cover; and, most seriously, drive-by-wire issues. My A6 4.2 was *SO* bad with regard to throttle response that my dealer bought it back (full price) even though it had 12k miles on it (!) lest they get sued if/when I got in a wreck. One of the other two we had was/is PERFECT, the other was so-so.
Despite the list above, *IF* you get one with no d-b-w problems, they are great cars. *I* think they're worth the occasional hassle and expense. But you just GOTTA get a good one. If the d-b-w acts even a LITTLE funny, even ONCE, pass on the car. It can't be fixed (my dealer tried everything imaginable).
Heh heh, if you get one, I have a set of new winter wheels/tires I'd like to sell.