I just bought this lens and made it out for about 30 mins until the heat killed me off (Dubai summer is horrid), here are the first images I took, so the AF is a million miles faster than my old Sigma 150-500, I'm well chuffed with my purchase
I went from the Sigma 150-500 to the 100-400L as well, the loss in focal length is well worth the gain in image quality, AF speed, handling and portability. Since I sold the Sigma, my monopod hasn't seen any use because I'm able to hand hold the 100-400L all day. I am hoping for an updated version though as it's getting quite dated by now.
By the way when I was in Dubai, one of the locals said Dubai has two seasons: Summer and 'Hell', so good luck over there