Re: Do You Rate Sigma Lenses?
Do I rate Sigma lenses? Well, yes and no I suppose is the short answer to that. My only experience with a Sigma lens relates to an 18/50 f3.5/f5.6 "kit" lens which I bought for my then newly acquired Canon 10D digital SLR body. Why did I pick the Sigma instead of the Canon optic? Well the answer was quite simple and I suspect may be a major influencing factor in many photographers decision to go the Sigma route..................PRICE. There simply was no equivalent Canon optic in that focal range at the Sigma price point. Yes, Canon did have a similar "kit" optic but it was only available in EF/S fitting and thus not suitable for my EF only fitting 10D body.
Perhaps I should elaborate on the term "kit" lens for those not familiar with the term but basically this is a lens built down to a price and often bundled as a "kit" along with a new camera body to provide the new DSLR owner with a "starter" lens. Why a "starter" lens? Well I suspect that a DSLR purchase is often a major drain on a camera purchasers financial resources and so the "starter" or "kit" lens is there as a "fill in" until a "better quality" optic can be purchased at a later date.
However, I digress slightly, back to the Sigma lens. How do I rate it? Well optically it was pretty abysmal at the extremes of aperture but started to "come good" when stopped down a couple of stops. To be honest, I reckon you could say that about almost any "kit" lens and I don't think the Sigma was any better or worse in that respect. Build quality I rated as pretty good actually with a nice finish and a "solid" feel to the lens. The AF was accurate and fast but noisy in comparison to a Canon USM lens but about the same as a Canon MM (Non-USM)lens.
Would I buy another Sigma lens? Well again I find myself answering yes and no. Yes, if money was tight and I couldn't afford the Canon equivalent and no if I could afford the Canon version. So why then given no financial constraints would I still go for the Canon? Well here we come to an important second factor in choosing the brand lens over the Sigma. It is my firm belief that the Canon optics are just that bit better in optical performance, construction and compatibility, particularly their professional "L" range of optics which are designed for and used by professional photographers. If a lens can stand up to the rigorous demands that a professional can put on an optic then it inspires confidence that it is a tested and proven choice for the keen amateur as well. This is the ace that Canon and Nikon have up their sleeves in that their own make lenses are used widely by professionals and it's one of the important factors that makes me believe that it is worth saving that bit longer and bearing the inevitably extra expense of the makers lens as opposed to the Sigma.
So in summary I'd say, close Sigma, but no cigar.........................when professional photographers start to use your lenses extensively instead of the Canon versions then maybe I'll reconsider my position, but until that day comes I'll remain a Canon man through and through......................funds permitting of course! 
BigWill
I'm sailing like a driftwood on a windy bay.