I recently got the autoenginuity device, and it is very nice. It has a database of manufacturer specific codes in addition to the standard OBDII codes, so that is very helpful. Having it based on the PC is nice since you have access to that database, and it has a lot of other functions a typical code reader does not have.
My son has an S4, and when he was home over Christmas the check engine light came on. We used the autoenginuity system to read the code and it had the Audi specific description of the error. It also allows you to monitor a lot of things in real time. His error was related to one of the O2 sensors on one bank, so we monitored that sensor in real time in addition to the same O2 sensor on the other bank, and it appeared to be giving roughly the same output (this is the twin turbo V6 S4). We reset the CEL, and it hasn't come back on yet.
I'm guessing many places would have found a way to charge an S4 owner a few hundred bucks for this "problem" -- perhaps it has already paid for itself.
Chuck