I use the 70-200 f2.8L non IS lens. It's a great lens but I'd love to have the IS version. Hey you can always turn IS off!
But here are the buts:
With a lens of this weight and focal length you are going to be better served if you use it on a monopod. I don't care how stable IS makes the lens, the IS won't take that weight off of your arms and the monopod will. And a monopod is a form of low tech IS, don't you think?
The other thing is subject movement. Even IF you can handhold this lens 3 stops lower than the non IS version, what is THAT shutterspeed going to do your subject? If it's a bird sitting on a branch, probably nothing; but if it's a kid running, or a car race, then the subject will be just as blurred with the IS as with the non IS lens.
From the outside it looks like the main utility of the IS lens is that you will be able to hand hold the lens under very poor lighting conditions. Get that "It's a slow shutterspeed lens." idea out of your head. My point is that in a lens of this focal length you are (should?) going to suport it with a monopod, and you are also going to use high shutterspeeds. Don't kid yourself otherwise. But if you DO use a monopod and 1/500 with the IS, you will get better images than with a non IS lens.
"There's never time to do it right. But there's always time to do it over."
Canon 5D, 50D; 16-35 f2.8L, 24-105 f4L IS, 50 f1.4, 100 f2.8 Macro, 70-200 f2.8L, 300mm f2.8L IS.