I'll be getting one of these new zooms from the Sydney distributor soon enough to test in the field.
600mm f/6.3 is fine in reasonable light, but not perfect and honestly, I only remember needing a short lens when I was photographing a sea eagle from a boat as it came in so close to take a fish we placed in the water for it. Naturally, I prefer a long lens of at least f/4 aperture for low light though f/5.6 is useable in case it is an 800mm prime.
For birds and wildlife, I could not honestly care for a zoom, as a long prime lens is my preferred beast to use and I can move to change my composition rather than use a zoom.
I'd much rather Sigma introduced a DO version of a 600mm f/4 OS or 800mm f/5.6 OS, but that is on my personal wish list. With the IQ of the Sigma lenses in the last five odd years, I think they would really make for great competition with the Canons and Nikons of the world for the serious wildlife shooter.
Incidentally, I have been reasonably happy when using the Mrs' 150-600mm contemporary lens as it does produce very good IQ, but it does not match the Sigma 500/4 Sport that is my main lens at the moment. Nowhere near it, so I am confident that the 60-600mm would be nowhere near the IQ of my prime lens either.