I recently took ownership of my family's '98.5 A4 1.8tQ. I had never really driven it before and was quite surprised with how nice it was to drive, especially given the mileage which is just shy of 100k now. It's very smooth and solid. No rattles. It's rather slow (compared to the NSX). Mine has 150hp, some of the newer 1.8T have 170. You can always chip it and get about 200hp out of it. I'm a bit scared to since i don't want to blow my 100k miles turbo. 98.5 cars have a bunch of updates over earlier cars. I think around 2000 there were a bunch of updates again. One of the nicer additions, imo, being the 1 piece projector headlights. The other reflector headlights look pretty crappy.
In terms of reliability, it hasn't been the best. I'm not sure if we've had bad luck or if they're just not that great overall. In general people say to be wery of german cars that are out of warrenty. They definitely break more than japanese cars and they cost more to fix. Mine decided to dump all its oil on the road the other day while I was driving it. Had it towed to the dealer (saw 4 other audis being flat bedded in at the same time, bad day?) and it turned out to be a gasket that had gone bad. $5 part, $340 repair bill. A month before that it dumped all it's oil too. The result of another gasket having gone bad. Before that the timing belt went at around 70k miles when they're rated for 90k. That made a mess of the valves and cost to $4k to fix.
So overall, nice car, be prepared to put some money in for maintainence. I'm sure we've had more bad luck than most, but things like keyless remotes, tie end links, gaskets, to name a few tend to go bad on the car.