Not sure on the becoming the new taxi's, not over here anyway, although Audi as a marque have taken off in the past 5 years down under so quite a few Audi's now floating around. Very few S4's overall.

Have had my S4 for just over a year and has been very good. It has all options except Sat Nav which by most accounts wasn't great with the Australian maps. Mine is the V8 and stock but wouldn't mind doing a couple of bits to it. Availability and choice isn't huge here.

I don't drive it much but it is superb when I do. Has been faultless and sounds superb. Few pics anyway: