I've been using the 50-500 combo with both my old Maxxum 5D (which I no longer own) and an a700. It is an excellent lens. I've sold many prints and images created with that combination. It's more than paid for itself. It's excellent.
But I'm still going to sell it and buy the 70-400G when it comes out.
There's a few reasons for that, even from a non-image quality standpoint.
1. The Bigma is huge. I'm sure you realize that. But the 70-400G is smaller and lighter.
2. Full time manual focus, focus limiters, focus hold buttons. These are all big features missing from the Bigma.
3. A better hood with a window to turn the polarizer with it attached.
4. The Bigma has an 86mm coarse filter thread. Using a CPL on it is expensive and difficult. The 70-400G has a much more common 77mm standard thread.
5. SSM focusing. AF performance on the Bigma is good, but not excellent. Speedier, quiet focusing is something I'd be interested in.
I have seen several samples from the 70-400G and image quality wise it's going to be as good as or better than the Bigma. From an IQ standpoint, there's nowhere else to go but up, especially wide open. At 400-500mm, the Bigma's vignetting performance is terrible. I rarely shoot at 500mm anyway, I use it more as a spotting scope. I'll gladly give up that 100mm for better performance all around.
Also, on an aside, there IS a Sony hand strap. But they call it a "grip belt." It also comes with a tripod socket adapter for the cameras without vertical grips. There's also the old Minolta HS-1000.http://www.sonystyle.com/webapp/wcs/stores...552921665295951