I'm a new convert to the A8 and just purchased a '04 8L 4.2. I drove BMW 750's for years and loved the car. I bought an '02 745 which the dealer had to take back after a year of not being able to resolve the various software issues. They gave me an '03 which was not a whole lot better.
In comparing the two vehicles, the 745 outperforms the A8 in most categories, acceleration, cornering, etc. However I find the A8 to be a competent, reliable sedan. The ride is a little soft, but when I lower the suspension to dynamic mode it stiffens up pretty good. This is my first Audi (I still have an '04 X5 4.4 and an '01 Z8) and I'm impressed with the car. Too bad the dealer is not nearly as impressive.
Jon