Well that's a very general question. How good in what way? There's a 2010 Toyota Auris HSD sitting outside my 92 year old father's front door that still goes very well and has never let him down unless he did something stupid like leave the lights on all night. Used regularly with some reasonable length runs and it will do 55mpg. I got 63mpg on the 300 mile run to deliver it. Absolutely no worries about EGRs and diesel particulate filters that plague modern diesel cars. If just used for short runs, the mpg goes down, but the car doesn't fail and doesn't suddenly turn into a mass polluter. Plug-in hybrids are another matter, but really only work if you have a regular 20 - 40 mile commute and can plug them in every night.