Sigma really knows how to exploit a niche, don't they! They have a way of evaluating the major camera makers' lens lineups,and then producing something that the camera makers are not producing--like a 17-70mm f/2.8~f/4 variable maximum aperture zoom lens WITH STABILIZATION for $450.
That is how Sigma got to where it is today...that and making lower-priced versions of popular lenses that the camera makers also make, like the 70-200 f/2.8 Sigma EX.
17 to 70 plus 70-200...Sigma looks to be going for a two-lens knockout punch!