See, I just can't understand how my 24-135mm Tamron bought used on ebay, for $250, is sharp out of the box on my 5D2, and my third copy of the 24-105L is like mush. For 4x the cost, I am sorry but this is just poor quality control. My 17-40L, 50EF Macro, is sharp and my 70-300mm IS is fair to good.
My dad during most of his life was a quality control inspector, and this kind of poor business model, not just with Canon or Nikon or whoever is just that a poor business. It is what killed the American Auto industry as a whole, but that is another conversation.
Manual focus adjustment, not for me, I don't have to time or the patience to do this, and frankly I just should not have to with a $1000+ lens. Good thing I love my 5D2 overall, cause if not, it would be going too. I would try a 24-70L but have heard the same issues with that. For now I will stick with my Tamron lens in a similar range, a known quantity, and a couple of fast primes and keep my fingers crossed. If Nikon ever releases a 24mp D700X, I have kept my Nikon lenses, I might just leave for good. IF I didn't need 20+mp, I might not have even tried the 5D2. If you are lucky enough to have a good copy of the 24-105mm L, be happy, as you are lucky, as I have not been so. BTW, it is not always the photographer who does not know how to use his gear, I hear this as an excuse and frankly that is very insulting overall, and if it wasn't too funny first.