The AF technology employed in the Canon EOS 1D X is completely different. It uses contrast as well as Colour detection. It even uses face detection. It also uses data from an AF correction light detection sensor (Digic IV) and with the new 400 f2.8 L IS II USM, 500 f4 L IS II USM, 600 mm f4 L IS II USM etc it uses the gyro sensors in the lens. The information from all this is processed in real time to give you a better auto focus than the predictive auto focus technology that was in use earlier. One should remember tha the amount of light at f8 is limited (especially from the light detection sensor). Limiting the AF to f8 is a deliberate move to get the best focus accuracy.

At the moment the only combinations for f8 were a 2x Tc coupled with an F4 lenses (500mm f4 IS or IS II, 600mm f4 IS or IS II and 400mm f4 DO) or 1.4x TC with 800 mm f5.6 lens. The 2x TC. Any other lens and a 2x will give you lousy results.

And one should remember that Auto focus at F8 was available with only the 1 Series bodies and that too with the central AF point. So If I have to make a harsh statement, not many of you guys would have used it and hence no point in bothering because of something which you never had. If anyone was using it regularly, then has to think how often it was and whether it was a do or die situation. Nikon guys never had this benefit. However, it is always good to have the ability to AF at f8 so that one can use a 600mm with 2x or 500mm with 2x or a 800mm with 1.4x. So now we have to live with this. If you think this was a must have situation, then better to pick up the 1D Mark IV till it is available.

I always suggest to use the manual focus to achieve critical focus. The long tele lenses always have the focus override facility so that you can fine tune your focus. I think it is better to use that or live view.

The only time I had tried using AF at f8 was with a Canon 300mm f4 L IS USM lens and EF 2xII TC and I gave up because I didn't like the overall quality. (I have to hunt where those images are). And That is also one of the reasons for buying the 400mm f2.8 L IS USM lens so that I can use a 2X TC and get 800mm at f5.6 and use all the AF points.

Shifting from 1.3 crop sensor camera to a Full frame camera will result in your subjects appearing smaller in the frame. And you guys using 1.6 crop cameras like 60D etc, will have difficulty in detecting the subject in a full frame camera. Frankly speaking, I don't expect the photographers who crop massively to post in photosharing sites will find this appealing (apart from the affordability factor).

The photographer or filmmaker who ever uses this camera will do so in combination with another body to reap the benefits of both.