Been thinking I'd like the 200mm f2.8 L for some time now as the 70-200 f2.8 IS L is usable but not fantastic wide open at the long end.
Still needed a zoom + IS so I thought I'd swap my lens for the 70-200 F4 IS + cash which I put towards a 200L.
Conclusion - good choice! If I need f2.8 I have the 200L - mega sharp, if I need even faster I have the 135L and for handholding the f4 IS in low light it is as good as the f2.8 IS as long as the subject is fairly static.
Both of my new lenses are sharper than sharp. The zoom is nice and light (I didn't have a problem with the weight of the 70-200 2.8 per se, but it was a bit overkill for wandering round snapping with the kids).
Thought I'd share my thoughts - pleased with my decision - hoped these thoughts might be useful to others navigating options around these focal lengths.