For many years I used a sigma 50-500 as my main (and for a while only!) lens. I had very few issues with image quality, and found it to be a tough and durable bit of kit. My son now uses it as his main lens on an old 350D.
Over time I covered the range with other lenses, but that is the expensive way of doing things (70-200 f2.8, 120-300 f2.8, 500 f4.5, and 1.4 and 2* TCs - all sigma). Dont shy away from third party lenses until you have tested them - I have found there to be very little difference between sigma EX and canon L series lenses, other than cost!

My one and only Tamron really has scared me away from non-canon lenses. However, I have been reading some reviews and the sigma does seem to come out ok.

I have a Sigma 50mm f1.4 which I ( and the reviews) reckon is better than the Canon equivalent, & a Sigma 85mm f1.4 which also delivers great results. Interestingly the Tamron 24-70 VC gets better reviews that the Sigma & appears to be almost on a par with the Canon which doesn't offer IS/VR

So as Gordon says - dont knock 3rd party lenses - read the reviews carefully - so of them are relative bargains :)

Thanks guys..I am saving my pennies now