I too have been considering these as well ad a 1d mk iv. I am an ameature who likes BIF and (sunset or lightning) landscape photography. I can afford the 1dx and the 600mm is ii USM with gitzo/wimberly. Not sure I should, but kinda my dream set up. 600 has enough reach and the 1dx tracking, 12 fps and low light all seem fun to play with. My 1st dslr was a 7d. This would be my 1st FF. I wish it had f8 focusing since I plan to use a 2x with the 600 f4. I would ultimately want something wider than 35mm eventually and maybe a 50 f1.2. Maybe the 24-70, but I'd prefer a wider aperature.