Same here! What camera? TC's? My next telephoto lens purchase is between the 400 f/2.8L IS II or the 600 f/4L IS II.
I had the same thought, but I decided that a 400 f2.8L IS II and a 1,4x Extender would be best for me.
This = 560mm f4. And on my 1D mkIV it will be 728mm, if my math is correct.
This will give me a more versatile lens with more options for cars and motorcyckles on the trackdays, sports and wildlife than just having a big 600mm.



































































