Gerard, I'm not saying Sigma are "cheap" but they are not built as rugged as the Nikon or Canon. every time one of my lenses as gone for repair in the last 10 years it as been a Sigma (and I had at least 6 lenses from them), the alloy used, its thickness, the use of plastic, zoom barrel coming loose are all components I had issue with, I only used the EX version of Sigma but had quite a few of them before I gave up and got the manufacturer version.
They do have a good warranty, at least in Canada its 7 years, but like any service, the best service is not having to need it.
I still use and love my Sigma 10-20, but its my third one, mind you, in one case I can't blame the lenses, it went down with my boat and spent two days underwater.
And now i'm looking into getting a Tokina 10-17 and expect more or less the same from it than I did from the Sigma.
BTW i'm no neantherdal cave man, but I used my stuff quite a bit and in the end rugged does pay.