I got my 70-300L a few days ago am report back with my experiences as promised. Generally, I'm very happy and would like to apologize for stirring up so much dust. But: better safe than sorry, esp. if a price it as the far end of one's budget.
I compared it with test shots at min. focal distance to my copy of a 100mm macro (non-L) which imho is as sharp as it gets on this 18MP sensor because I can count the pixels when using f6.3-f7.1. This 70-300L is just as sharp and much sharper at open aperture. Looking at my "real life" shots, I can only concur to this observation - if I can see every feather of a bird in detail, that's enough for me. Even the af is spot-on without afam on my 60D body, but that might be easier w/ f4 than w/ f2.8. It does show some CAs at high contrast borders and open aperture, but these are easily corrected by software.
Before anyone freaks out: The 200-300 range does not make that much difference and I would be tempted if anyone would swap this for a 70-200/2.8is2+1.4ex or 300/2.8 for free - but that's not likely to happen :-). My other impressions:
* The lens is still balanced w/ my 60D, but I guess a 7D might even be a little better. Because of the short lens size, the weight is near the body so it creates little torsion. If a lens gets any longer and/or heavier I would insist on getting paid for this. Even after one day out my right arm feels a little longer - and considering this mass, it might work as a self defense weapon when being attacked by a wild boar.
* This might come as a surprise to some: I like the reversed af/zoom rings better, because I can override the af at the point where my middle finger is on top of the lens anyway. I don't see how I'd accidentally defocus it with my current technique. Arguably, the zoom ring is not reachable with the lens hood on reverse, and zooming requires hand movement - but I generally crop the pictures afterwards anyway, so setting a zoom length and then using it like a semi-prime is just up my alley.
* This won't be a surprise to most: The build quality and IS is stellar, and while I would generally prefer an internal zoom like the 70-200 lenses this 70-300 is so short that it still fits in my bike bag I'm always carrying around. It is not a "shoot moving objects in the dark" lens, but it does what I'd expect from a red ring: it has top performance at open aperture. So if an objects is stationary for a moment, it works even for wildlife at lower light as long as the dof of f4-f5.6 is not too thin.
My positive conclusion: I still don't own a big white lens, it's a big grey lens :-) ... and for aps-c and my preferences, the size-weight-iq-af-zoomfactor-buildquality-price combination and tradeoff is just right. If I ever need a faster lens with less dof, I'll get a really fast f2 or less prime with the money I saved from skipping the 70-200is2.