They say that the A8 is the compromise between the S classe and the Bmw 7 series, because it can be very comfortable and if you want it can also be very sporty. The S classe is however the most comfortable car to drive (they say its the king of comfort). The 7 serie is as always very sporty ans dynamic in steering, and is 100 kg lighter than the Audi(Audi weighs 2070 kg, the Merc weighs 2050 kg). The 7 serie also has a better fuel economy than Audi and Merc, it consumes 0.7 liter less than Audi, and 2.4 liter less than the Merc.
The Audi A8 however can also be driven comfortable or sporty whenever you want and is the quickest car to reach 100Km/h, in 5.5 seconds (BMW and Merc are 1 second slower). It has a lot of settings for the drive pleasure. So what they say is that the Audi A8 is a very good car, but it also has its price. The A8(4.2 tdi) starts with +10.000 euro more than the BMW 7 serie with its basis price.
BUT in the end (I think) they say is that the Merc and BMW should watch out for the A8 because its such a good car, and I think they find the A8 the best car of the three.