David - you are basing that on what lenses? the sigma i sold you was an old film lens - and i'm fairly sure you dont own any high end digital sigmas.  Have you had a look through the sigma 70-200?  Also the recommendation to me for the 50-500 came from a pro photographer who owns L-glass... (and incidentally the lens has a higher user rating than the tokina you recommended on the site i quote below). A sweeping statement of "dont use this manufacturer" isnt really productive or objective...

Anyway - besides personal experiences with lenses or manufacturers, for an objective look at various lenses look at this site for any further info on the majority of lenses out there - it has multiple user reviews, not a lone "expert" (which is basically all me and david are in this thread) - and is a good gauge of how a lens performs as two isolated and conflicting ideas about a lens' performance arent going to help:

http://www.fredmiranda.com/reviews/

The 170-500 isnt on there but here is an interesting forum thread which probably you should take stock of:

http://forum.warehouseexpress.com/message.asp?FTopicID=1509

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography