I've had the 170-500mm for a couple of years now, not the DG version.
It's worked well with Digi and film. I haven't had any problems with CA/PF.
It's a bit soft at the 500mm length, nothing that can't be cured with sharpening.
The older the lens gets the more creep I have in the zoom, I work around this
by keeping it level when carried. There's a reveiw out there somewhere that compares it to other lens, C.n.n I think. It holds it's own. If I wear this one out
I'll probably buy another.