I used to have the 70-200 F4L non IS and now i upgraded to the 2.8L IS
out of all 4 of the 70-200 L lenses, the F4L non IS is the sharpest.
In my point of view, I think the IS on the F4L is unnecessary. The non IS is much lighter. IF there is a budget constraint from getting the 2.8L IS, you might as well to consider F4 or F2.8 non IS. I believe 2.8 non IS is only slightly more expensive than f4L IS