Used with an EOS 30D. I wanted a good walk-around trans-standard zoom, easy to use with suitable preformances. Nothing to complain with this small affordable one and I am very pleased with.
Sigma's sharpness and speed are nothing than correct.
AF is not USM but works well, speed and accuracy. A little noisy but better than my other zoom Tamron, which is quiet also all said. I quite never need to go to manual mode, except in darkness. In this situation, AF ring too loosy and short course is a mess, that's clear.
Distorsion, which distorsion ? Mine shows very low, I'm happy.
Colours are good, maybe a little warm but easy to correct with camera settings.
Major weakness is the vignetting which is very, very, very (did I say very?) visible at wide end and large aperture.
Built quality is more than descent. The zoom ring was harsh when new, now getting smoother and confortable to use. No wobble. Dust protection seems good, better than with non L Canon zooms in any case.
The size is just as needed. Only the focus ring which turns when adjusting AF and requests some attention. Nothing important with practice.
Last point, the price. I think nothing to compare with at this level of quality + Sold With sun-visor.
I don't know about Tokina. Lots of people saying they produce nice objectives.
Canon EFS 17-85/F4-5.6 distords too much ... and CA! EFS 18-55/F2.8 very good but lot more expensive and heavy and bigger. EF 17-40/F4 too short in zoom range.
I think only competitors are Sigma 18-50 and Tamron 17-50 both F2.8 but both have their own weaknesses and shorter long end.
Get the oportunity to test Nikon D200 + 18-70/F3.5-4.5 before buying. Except sealing and VR AF (equivalent to USM) I found everything to be equal (sharpness) or lower (CA, distortion). And the price !
So, at the end, I recommand it for non pro photographer. In any cases I think it is fare better than kit zoom !