The 70-200 2.8 IS-USM loses some sharpness at the longer end, particularly when shot on an APS-C Canon body...it's an old enough design that it's really "best" on FF. That's why Canon is re-designing the lens--the newer f/4 IS design optically is a bit better than the older 2.8 IS model. It's not like it's a pig or anything, the 2.8 IS USM--it's a very solid lens, especially on FF. But it does taper off in sharpness beyond about 140mm. However, around the 135mm mark at f/6.3 or so, it's simply amazing...
If I were going to buy a lens for use on a really,really high-density,smaller sensor like the 7D, I'd go for the newest lens that delivers the absolute highest MTF--and right now, that's the f/4 IS-USM version...and probably the "new" Mark II f/2.8 model. I have not seen the new 70-200 2.8 Mark II's MTF charts or any samples from it.
The new Sigma "could be" a great lens; it uses Sigma's newly developed Fluorite-Like Dispersion glass, which could resolve chromatic aberration issues at wide apertures like the all-important f/2.8 to f/4.5 zone where older, less-ambitious lenses like the Sigma 70-200's have not been as good as Canon or Nikon high-grade $1699-$2400 lens designs. Sigma has not yet announced the price of their new 70-200 2.8 OS lens; it might be higher, like $1699, and it "might be" kick-ass, like their 150mm macro.