The 550 is certainly a gas hog. I've only driven mine roughly 1,000 miles, but I seem to be averaging 18.3 mpg, 350 miles to a full tank. 70% highway perhaps. This is in contrast to my 2002 540, which averaged 22 mpg. 100 extra horse power = 20% reduction in fuel economy. Hard to explain in a town where the Prius rules.
But the ride justifies the guzzle I suppose.