Sigma 170-500 mm f/5-6.3 RF APO Lens

I purchased this lens in March of 2007 and have used it EXTENSIVELY. After reading alot of other reviews (all over the internet) I kind of felt that this particular lens was not getting a fair shake, or possibly that I got a great deal on an extra-sharp copy. Whichever is the case, I have to disagree with alot of the bad reviews. My photographs have been sharper then any other lens I have used at the 500mm end including the 50-500mm sigma (bigma), the exceptions are the 500mm and 600mm canon L lenses I rented for a day.
Some things to remember about this lens, it does need alot of light, I rarely take it out on cloudy days.
Use of a tripod, or some other stabilizing device (I use a Brushawk 310D) is important at 500mm.
Lastly, I have used this lens with a Tamron 1.4x converter and it has worked flawlessly (again, it was a bright day).

PROS
* A great range at a GREAT price (running about 400-600.00 here on Ebay)
* Sharp, when used properly.
* Built really strong
* great telephoto adjustment and focus rings.

CONS
* Sealing would be nice, though dust has not hurt any of my shots, I use it enough that eventually it will.
* IS (or sigma's OS) would be really great, but would run the cost over 1000.00 easy.

Overall I would have to say that this is a GREAT lens with great range and for the price it is darned near unbeatable for a semi-pro in need of a long range lens.