I have been using the Sigma lens for the last 10 months or so, replacing my canon 100-400L as my main nature photography lens. I mainly use this lens on my 1D IV with a few shots here and there on the 5D III.
This lens is very sharp, my version has fantastic centre sharpness even at f2.8.
I can use both 1.4 and 2.0 teleconverters easily still getting sharper images than my old 100-400L (definitely not as sharp as the latest canon 300L with or without the teleconverters).
Autofocus is slower than any canon L series lens and almost nothing compares to the canon 300L, but tracking autofocus on the 1D IV even with a 2X teleconverter for birds in flight is still very good. If the lens does lose focus it can hunt (I think this is better on the latest version with focus limiter).
Autofocus on my version is spot on for single shot.
IS works very well, especially with the teleconverters on.
Not so good things:
The weight is very significant for hand holding. I have gotten used to it but the 300L is much easier to handle.
Weight and size are not so good for travel, I do considerable treks into mountains, woods, where ever and my neck and back pay for it.
No weather resistant properties is also a down side, but I knew that when I got it.
From my point of view this lens is more useful most of the time than my old 100-400L, and is pretty much in between the price points of that and the 300L. I think it is more useful than the Sigma 300 prime at nearly the same price. Nothing I have seen beats the Canon 300L in sharpness, autofocus speed, and most of the other features, but I was able to pick up the Sigma used for $2100. That extra $3-4000 for the Canon is still difficult for me to justify, at least until I can make enough off of my nature shots or hit the lottery (likely both about the same odds
I may change my tune if/when canon updates their telephoto zoom.
Here is one of my latest shots tracking a Black-crowned night heron using 1D IV, Sigma 120-300, canon 2X teleconverter.Red eye flight
, on Flickr