Originally posted by cgoudie I don't know that I can broadly generalize unless we're comparing lenses of similar focal lengths.
I have the Tamron 90mm/F2.8 Macro, and I love it's pictures, but the lens just isn't as well constructed as my Pentax gear. When I pull it out of it's bag, especially if the lens cap is on, the lens extends to the end of it's focus. It just feels less solid than my other gear. The fact that it's always got the AF screw engaged drives me nuts. All that said, its pictures are stunning. It got a highly recommended from photozone.de, and for the $350 I paid for it used, I'd say I'm getting way more than my moneys worth... But then again at the time, the Pentax WR macro was fresh out and $900.
I don't have any Sigma lenses yet, but I looked at an older overpriced Sigma 50-300 lens in a pawn shop a week ago, and I was unimpressed by it's build quality as well. It felt plasticly.
I'm certain that higher end Sigma gear would impress me more than the kit-ish lens I was looking at.
most of the older Sigmas as garbage. the recent EX build look tough and feel solid compared to the build of modern DA lenses (kitlenses ,16-45 , 55-300, 12-24). they are also much bigger, wider and quite heavy as well. as for IQ, I could attest to the Sigma 70's brilliance.