This is a fine lens. I have to state that IS is of significant advantage and noticably increases the percentage of keepers.
I use this lens with the Gitzo 1548 and Whimberley Gimbal Head. One feature I really like is the ability to remove the lens foot and replace it with a Whimberley Plate.
I primarily use this lens for Wildlife. I also have a full size vehicle door mount that supports this lens, Whimberley, flash and Better Beamer Flash extender on the exterior of the door.
Since purchasing this lens, I have to say I have added to my portfolio much faster than when I was using a 400 mm. I find myself rarley wanting more focal length than the 600 f4 + 1.4X Teleconverter.