I've tried all three current options. The tiptronic (only on the 3.0 TDI) is the best suited box for the car, the smoothest/most refined drive. The S Tronic when driven in comfort mode is pretty good, and obviously can be driven "manually" when the mood takes you, not quite as smooth as the tiptronic, but a good balance of "sporty" and "wafty".
The manual on the other hand, whilst "fine", never offered the relaxing drive that cars in the class of the A6 are good at delivering, it just doesn't make any real sense. If you want to drive "manually" knock the S Tronic into manual, because for most of the journey, or on the motorway, or in stop start traffic, all the basic manual becomes is an inconvenience, it adds nothing to this class of car.
The manual on the other hand, whilst "fine", never offered the relaxing drive that cars in the class of the A6 are good at delivering, it just doesn't make any real sense. If you want to drive "manually" knock the S Tronic into manual, because for most of the journey, or on the motorway, or in stop start traffic, all the basic manual becomes is an inconvenience, it adds nothing to this class of car.