I had a 70-300 DO several years back and loved the lens. I wanted something with good reach but as lightweight as possible as my wife was using the camera and couldn't hold heavier lenses. It was a perfect walkaround travel lens and it felt quite compact. But I soon wanted something a bit faster and to be honest a bit sharper. The DO behaved a bit differently to normal lenses and for me needed more processing to get the images to sing. Also it produced odd donut type bokeh, which drives some people bonkers but I didn't mind. The lens is actually quite unloved by many, and I was one of the few who enjoyed using it.
However for all the advantages I thought it gave me, it eventually went to fund more conventional lens purchases. That was my main gripe, it was just too expensive and I ended up with a 70-200 L F4 is plus 1.4x tele for less money than I originally paid for the DO.