from my experience with the 70-200 F4, it's a wonderfull lens, and nice and sharp, the IS is nothing short of incredible at 4stops equivilent.....
that being said, it's the same IS system on the F2.8
I picked up the F2.8, and and selling the F4 to my friend if he decides he wants it (pending at this time)
The weight of the F2.8 is a huge down point, but as i'm a big guy, I never really notice the wight untill i had it to someone half my size then see them almost drop it.
that's when i realize, i don't really need to hand that camera over to anyone to play with.
The F4 was so light and easy to maneuver and is actually shorter in length.
It is too early at this time to really do a comparison between the 2 as i'm still getting used to the newer one, but i find it has a lot of vignetting, which is sometimes fun, but i usually end up pulling most of that out of the pictures.