500mm vs 600mm?

I bought a 500 as an upgrade to my 300/2.8L II. I was always very happy with the 300 but I used it with the 2x III 95% of the time, and although the image quality of the combination is very good I felt I was missing too many shots due to the AF performance hit. I considered a switch to the 400 DO with a 1.4x to keep the small size (and even less weight), but in the end the extra reach of the 500 was just too strong a pull.

I didn't get as far as thinking about the cost difference between the 500 and the 600, because I was determined to stick with a lens which I could hand hold if necessary. I do mostly use the 500 with a tripod and gimbal head, bought for the purpose, but it has been very useful to be able to hand-hold it some of the time, in a whole range of circumstances. Also it is perfectly practical to carry it with a shoulder strap (Peak Design Slide - highly recommended) whereas most people I know with 600s have had to buy dedicated bags and pack the camera away to walk anywhere. Having said that I do know a couple of people who walk around with 600s ready for action so if you're strong enough it can be done.

So in my opinion/experience, the 500 is a much more practical proposition than the 600. I wouldn't want the extra size and weight, to say nothing of the cost, for just 20% increase in reach - only half the amount you get from a 1.4x Extender.

There have been rumours of a 600DO, maybe next year - now that would change everything... but I wouldn't expect any change out of £15K