In reply to balmybaldwin:
I have owned two sigma lenses for a Nikon mount and I have to say that they outperformed the Nikon branded lenses of roughly the same price and spec bracket.
I can not comment on Canon kit, as I have never owned any but I can happily state that Sigma lenses never failed to impress me on image quality.
I have now moved away from Nikon to Panasonic micro four thirds. There obviously isn't the same size of variety of lenses going back decades. However modern technology and materials help make far superior glass with fewer defects and better quality all round.
I'd even hazard a guess that any modern lens designed from scratch by any of the reputable companies will outperform older glass built years ago.
Personally, budget allowing, I'd buy the most modern designed glass that I could afford rather than something a few generations before it.