I had a chance to test drive both X5 3.0si and 3.0d. No matter how much diesel engine technology has improved so far, you cannot compare the refinement, smoothness and responsiveness of the gasoline engine with the diesel engine. Diesel is "fine" by itself, but rough idling and coarse sound (all relative to the gasoline counterpart) makes it apparent that it's a diesel engine. Of course, driving a diesel is all about low fuel consumption, and you can't argue with that! And strong torque is a plus.