I'm getting this lens tomorrow! It's been on my radar for a long time. Had the 70-200 f/4L (non-IS) years ago, my first L lens, and loved it. Was gonna get a second copy of the Sigma 180mm f/2.8 OS macro (I got one last month but it was faulty), but decided for a bit more money, this lens would be more versatile. Exciting!
What problems did you have with the Sigma? I replaced mine because the hoods were very loose and there was poor fit between the lens and camera body. Second one fit the camera body better, but the hoods - not.
Oops - I better comment on the 70-200 2.8! Great lens, just great.
Haha it's okay to go off topic a little. There was something wrong with the focus ring. Autofocus and manual focus failed in some situations, like the ring was decoupled from the mechanism or something. I just got a refund. The image quality was superb when focus could be achieved, however.
But the 70-200 has been on my wishlist for a long time. I'll see how it works paired with extension tubes and extenders. I don't need it for true macro work mostly, but near-macro, like flowers, large insects, etc.
Current equipment: 5Ds, 5D mark III, 50D, 24-105L, MP-E, 70-200 f/2.8L II, 100L macro, 500L IS II; 1.4xIII + 2x III extenders; 600EX-RT.
Former equipment includes: 300D; EOS-M, EF-M 18-55, Samyang 14mm f/2.8, EF 35 f/2 IS, 70-200L f/4 non-IS, 85L II, Sigma 180 macro, 200L 2.8, 400L 5.6