They're as reliable as any other Audi from the same era... Which is pretty ****** reliable! Depends what you want to use it for I suppose?
My daily (TDI PD115) has just ticked over quarter of a million miles and has never been better... Drives well, goes well, starts on the button, take a bit of abuse every now and again and still returns 55mpg!
I think the best buys all round would either be a PD TDI or 1.8t... V6 is nice but just drink too much and a 1.8t can be easily tuned to more than a v6 anyway and still return decent mpg's