The 200mm f/2.0L IS USM seems to be praised as the sharpest lens Canon makes. It also is very fast at 2.0. For a few more days I could buy one with a $500 discount. Many photographers seem to opt for the 70-200mm f/2.8 L IS which is about to be updated to a new "II" version with claimed improved optics, shorter focusing distance, faster focusing, better IS than the current model. The zoom offers zoom flexibility and less weight/bulk - but is slower and I presume the optics still won't nearly match the fixed 200mm.
I find the 200mm focal length useful for sporting events where I can be relatively close to the action (the fastest lens would be useful here because they often don't occur outdoors in the sun) and capturing closeups in group settings.
Decision time - grab the fixed lens for $4,800 now or wait for the new 70-200 which I would guess will be north of $2,500. Would I see a difference in pictures taken at 200mm? Is the f2.0 versus 2.8 a big deal?