That is pretty bad for a hybrid, and pretty ugly.
But 20MPG combined is on par with its 'competitors' The S550 is 18MPG and the 740i is 20MPG as well. Only the A8 is better at 21MPG.
I dunno, I think for a 5600lbs getting the same mileage as ~4500lbs cars isn't a total failure.
The real failure is making the car so damn heavy to begin with.