it is the modification and re-certification process that's holding back widespread use of diesel engines but the advantages are clear; cheaper fuel, better consumption, less engine maintenance and no more mag drops. i've heard figures as high as $30,000 to convert a standard certified aircraft to diesel and that could be more than the aircraft's actually worth. As A&C says, it's still a long way off from being the norm unless you can afford that fabulous Diamond TwinStar!