I have the VAG-COM tool and I find it very useful. I don't know what all it can check on the V8 models, but it is every bit as good as the 5051 computers the dealerships use. For basic fault checking you don't need additional documentation, but if you want to do the advanced functions you need to know more information about different codes and advanced data channels. This information can be found in service manuals by Robert Bentley Publishers. I've used VAG-COM on my VW TDI and it works very well, but it won't talk to my early build 1990 Audi 90Q 20V.

-Brian
3.6L Audi 90Q in progress...