In article , David Dyer-Bennet
wrote:

That's OK. Most of us rarely take Sigma seriously. Except for the
occasional good lens, most of their stuff is crap.

Sigma is one of the most aggressive lens companies lately, and has made
some outstandingly good lenses. I've never had a bad lens from them.
(Then again, I've chosen all my lenses, from them and others, after
considerable research.)

you must be incredibly lucky. most people need to go through 3 or 4
copies of a sigma lens to get one that actually works properly and is
not decentered or has other problems.

as for aggressive, i suppose you could call it aggressive when they
stole nikon's stabilization patents to use it for their own lenses.
unfortunately for sigma, they're being sued by nikon. they also stole
canon's ef protocol for their own sigma mount lenses. nice honest
folks. not.
http://www.imaging-resource.com/NEWS/1306345284.html

(Happy owner of Sigma 105/2.8 macro, 12-24/4.5 full-frame,
120-400/4.5-5.6).

sigma macros are mostly ok, however, the 12-24 is not that great. the
120-400 is junk, as is its almost identical twin, the 150-500.

lensrentals ceased carrying most sigma lenses because of constant
problems. the 120-400 and the 150-500 had a nearly 50% failure rate,
but at least you didn't get the 120-300 with its very impressive 90%
failure rate.
http://www.lensrentals.com/news/2008.09.20/lens-repair-data-10