I love my Sigma 10-20. Sometimes it produces rather soft results, but I put this down to my 450D's abysmal ability to focus properly except in live view. The following image was taken with my Sigma, from a very harsh crop. It was also one of the few images that had focussed properly in quick-AF:
That hasnt been sharpened either IIRC.
The lens feels quality (much heavier than the 18-55 for instance) and is extremely practical. For the things I use the camera for, 'stepping back' is simply not an option at times, and since I've always had the urge to 'step back' ever since i started using a PnS. This lens makes that true.
here is a photo taken at 35mm, standing at my bathroom door:
(Let's bear in mind that 35mm is the wide end of most PnS and bridge cameras)
and now for a shot at 10mm standing from exactly the same place:
One thing that I did notice with the Sigma, is it is a very WARM lens. This photo of our test subject, Jack, was on auto white balance:
AWB usually sorts 99% of my shots out with the 18-55, but the Sigma needs a good bit of tweaking at times and often delivers warm results. Vignetting is present as well as you can see at 10mm, is marginally better at 14mm and is almost gone by 17mm. By 20mm there isn't really any noticeable vignetting.
Lower than 16/17mm, the onboard flash of my 450D (and other EOS cameras I suspect) was not sufficient to fully light up the area that the Sigma covers - and at the widest ends you see a shadow from the lens. However - this can be used to an advantage. I quite often use my onboard flash, with Siggy, at 10mm, to only light up the area I wish to draw attention. Throwing the rest out of focus and out of importance, the eye is really drawn to the subject. See:
I'm not a professional - far from it - but the Sigma 10-20 has brought out a creative streak in me. It's own limitations are by right its advantages as well, and the possibilities are almost endless. Yes, I have been informed that the Toki 11-16 is sharper - I personally haven't tried. Unlike some folk on here who've probably had one go of the Sigma, already made their judgement and constantly bang on about how sub standard it is, I'm not going to bang on about the other lenses which I have not tried. But. I will say, I LOVE MY SIGMA!!!!
As a side note; the 10-20 does not have IS, but at those focal lengths, it doesn't really need it. Also, the f4-5.6 version is rather slow, but again, a tripod and a longer exposure can sort that. Don't try getting bokeh off this lens, for it just won't happen. The slight fuzzy effect of infinity-distance backgrounds is about as best you'll get.