Ok, my 2 cents.
I too tired of waiting for the 560mm and have no regrets on my Sigma 500 so far. The build seems a little flimsy to someone coming to it from the A*400/2.8 howitzer but when I realized i could actually carry and shoot it hand-held I was thrilled. Sometimes with a carbon fiber monopod but in a pinch its not too heavy to hold and unlike DadiPentak, I am no Charles Atlas.
Filters; it comes with a clear and a CPL which you can rotate from outside the barrel. Although the filter is part of the optical formula (take it out and what was sharp is now blurred), I see no difference when using AF with or without filters so I never use them.
Balance seems pretty good with an arca-swiss plate that runs the length of the grip and balances evenly with a k-5+grip+TC+flash+betterbeamer.
I picked up a new Sigma 1.4x TC that mates well with the lense and although its advertised as MF only, the AF works just fine for me.
No complaints on sharpness although might not be as good as the A*400/2.8, I have some bird shots, especially the close ups at 5m, where every single fiber and fine hair is sharp.
My only negative on it does not affect shots and that is the build does not seem like a $5k piece of machinery. But the shots do. Unless the Pentax 560mm is a lot less than $5k, it will trouble competing with the f4.5 that autofocuses with a TC at 700mm.
--end of 2 cents
edit - Dadipentak knows more about the Sig 500mm than the designers. Lots of shots posted in the 300mm+ thread.
Last edited by imtheguy; 07-26-2012 at 08:07 PM.