I'm still driving my chipped B5 S4 and was thinking about moving to the V8.

Audi has historically had reliability issues (electrical gremlins, etc) that they continue to work on. Late 80s early 90s, I would've never purchased an Audi. That said, it appears they've put much of that behind them

I've had my 07 S4 for a year and knock on wood, I haven't had any problems.