Let me start off by saying that if you are shooting on a budget there is much more to consider than the price of a lense. While I am extremely happy with the quality of captured images and the versatility it has afforded me through many expeditions abroad, I will never buy another Sigma Product.
As stated, I own the 50-500 and had loved every minute of it, but recently the autofocus went out on me. So I took the lense to the local camera store, had it inspected and sent in to sigma. Well NINE, count'em 9 weeks later I finally got my lense back from Sigma. And believe me; this was not without fan fair or a threat of legal action.
That's the short version of a long convoluted story..
In closing I'd just like to add that I have had to send items in to Canon, HP and others and received them back within a week or two. If Sigma wants to sell lense for use by enthusiast and professionals then they should be prepared to treat us as such, and provide better service.
Comparable Canon glass may cost more financially, but the stress induced by Sigma's incredibly inferior customer service, makes the price of Canon glass and excellent customer service a no-brainer.