I routinely get 40-45 MPG on the highway in my 2003 VW Jetta TDI with an automatic transmission while driving between 70-80 mph with the air conditioning running at max. I have never had a problem with diesel fuel quality or availability.
My choice of a diesel was not simply to achieve a lower fuel cost per mile. Maintenance costs and reliability are also better due to the lack of certain systems within a diesel engine (i.e. no spark plugs, alternator, etc.).