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The higher end lenses I've used from Sigma have a similar failure/bad rate to Canon units. The less expensive/kit lenses from Sigma have at times been maligned partially from inexperienced users or from those expecting to get something for nothing. Also they are not as sharp at maximum aperture as they are not designed that way as are many of the Canon 'L' units. Then again there's an order of magnitude difference in price in some of the lens units as well!
There is the consideration the Canon EF-S will not fit on some older cameras that the Sigma apparently will.
Sigma is quite responsive to warrantly issues , at least in the US, and will repair or upgrade chips as needed to make their gear work.