Sigma lenses seem to be very hit and miss. Many mixed reports.
I've never actually kept one of their lenses longer than a week before taking it back to the shops in annoyance for a refund. There's been a scratched internal element, serious dust, or I just hated the lens altogether. The 'EX' build quality label is nothing compared to the build quality of a good home-grown lens from Canon, Nikon or 'Minolta'.
I can't comment on the 170-500, but the 50-500 I had was as sharp as you can expect such a lens to be, quite usable, even with a badly scratched internal element. It had to go back becuase there would be no resale value with such damage, otherwise there was no problem... except the fantatic weight. I'd rather have a Nikon 80-400 lens than a Bigma, it's so light and airy. The Minolta 100-400 was a good lens, bit slow to focus.