My decision was between a Mercedes S550 4 matic sport and the Audi S8. The S550 is nice, but the S8 just felt like something more special. The v10 engine (smooth and quiet at normal speeds, and unbelievably poweful and "baritone" when you want it to be), and the special "S" interior and exterior felt like something a step above the norm. Even the ride (with the std 20 inch rims) is wonderful in comfort mode... unbelievable given the potholes in my part of the country). I average about 15.5 mpg (70% city/30% highway). Not great, but hey, that v10 power/noise has got to come from somewhere. Net, no regrets so far.
Fun to drive. Incredibly fast when in "S" mode. Comfortably seats 5. Great interior craftsmanship. Even my wife said she could see what was "so special" about the car. Definitely consider Certified Pre-owned if available because potential repair costs would be astronomical.
This is the most outstanding car I've ever driven. I also own a Mercedes SL600 (with a Kleemann power upgrade) and in the morning, I find myself reaching for the S8 keys. This car is a very fast sedan, but what I really love about it (besides the all wheel drive for Wisconsin winters) is that it is so comfortable to drive and that the dash layout is so simple and intuitive. This is the first sedan that I have ever found to be exciting to drive. The paddle shifters are way cool.
After 3 years in a Porsche 911 Carrera S, I decided I'd been battered & deafened long enough. The S8 is the perfect replacement - a coddling, comforting living room when you need peace and quiet. But when you need to scream, it will gladly scream for you. The baritone bark of the V10 is unrivaled by any other luxury sedan, and the acceleration is phenomenal, with quick "whomp whomps" of the paddle shifted transmission. Yet, unlike the 911, the S8 feels utterly stable -- no wiggly sense of instability or choppiness. On the highway, it is impeccable, quiet, yet ready to blast into triple digits at a moment's notice. It is subtle looking, yes, but people still notice it due to rarity.