IS is valuable for panning at slower speeds. Really helps quell that up-and-down movement that ruins slow-speed panning shots. As far as a 70-200 f/2.8--my advice is make it a Canon, not a Sigma. And if you shoot Nikon, make it a Nikon, not a Sigma. The single biggest differences between the Sigma 70-200's and the Canon and Nikon models, in test after test, seems to come down to three absolutely critical areas. 1) Better optical quality in one,two,or three metrics with the lens wide-open with the camera-maker lenses, which are more expensive. 2) Faster autofocusing with the camera maker lenses; with the Tamron 70-200 for example, its focus is dog-slow compared to a Canon L-series or a gold-ring Nikkor 70-200 and 3) HUGE resale advantage for the camera-maker lenses over the 3rd party lenses. With the 3rd party makers like Sigma most specifically, recently trying to DOUBLE, or more, their retail prices on higher-end lenses, it now makes little sense to buy a 3rd party lens because the original price is so high and because the depreciation on the used market is so rapid with the 3rd party lenses.
I love panning. I have shot a lot of pans without stabilization, and now, in the 2000's, WITH stabilization. I personally feel that Nikon's auto-sensing, dual-mode VR is the best system for panning ever invented. The Canon Mode 1, Mode 2 system is a bit less handy, but it works well on my 70-200 2.8 L-IS, which is the 'original' model, and not the new Mark II model. Not sure, but I thought Canon updated their IS system to an auto-sensing, auto-switching system similar to what Nikon uses in its Mark II 70-200 version; regardless, I honestly believe that lens-based stabilization is the BEST thing to ever happen for people who like to do slow-speed panning shots. The ratio of keepers to duds now that we have stabilized telephotos and zooms is much more favorable than it ever was in the 80's and 90's when doing slow-speed panning shots.
A 70-200/2.8 is a workhorse lens that should last 10 to 15 years, or more. If the lens is decidedly poor at f/2.8, it's a waste of money. The newer Canon and newer Nikkor 70-200 top-line models really kick the a$$ of the 3rd party lenses at f/2.8
So I guess I'll be looking for 2,8 without IS since I only shoot on slower shutter when I'm trying to do some panning and in that case I usually can get close enough to use my 17-70 which has IS. Tyvm for your advice! 
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