I used one with the 300 f2.8 vr1, I was impressed with the quality considering it was a 2x teleconference. It's something I've missed since selling the 300 f2.8 and 2x mk3 teleconverter, it was a nice combo for birds in flight when I needed the extra focal length.One of only 2 lenses i would bother adding it to and the other us a 300 f2.8
Does it really add anything on an 'older' lens (non-'F', without the electronic diaphragm and fluorite coating)?
Most reviews I've read say there is no noticeable difference to the MkII.