One particular advantage of diesels in US conditions/expectations is that the penalty for driving a larger vehicle is reduced. The UK consumer magazine has just done some real-world fuel economy testing on eco-cars and they confirm this size issue.
The latest BlueMotion (ie, most economical) VW Passat achieved 45mpg average in their tests - exactly the same mpg as a Prius and slightly better than a hybrid Civic (43mpg) - whereas the extreme BlueMotion (eg, no spare wheel, no air con) version of the Polo (one size smaller than a Golf) got 50mpg. So it only 'costs' 10% to go up two sizes in a diesel, which would be more like 20-30% if they were gas-powered.
Some other real-world figures are 44mpg for a diesel Mini, 41mpg for a diesel BMW 1 series (one size smaller than a 3 series) and 45mpg for a diesel Citroen about the size of a Golf.
All these mpg figures are in US gallons, but they would all be better in the US where the driving is much less stop-start than in Europe. The government test mpg figures for these cars are all about 20-25% better than these 'real world' figures.
Andrew