French site lemondedelaphoto (translated) reviewed the Sigma 24-105mm f/4 DG OS HSM lens, Sigma’s latest addition to the “Art” line-up (in stock at B&H for $899). Please note that the links to the pages of the review are not working when translating with Google (at least not for me).
Optical quality measures are better than Canon’s equivalent, the EF 24-105mm f/4L IS (price & specs). The reviewer outlines the very good build quality, and auto-focus performance. They observed vignetting at 24mm and distortion (though figures appear to be better than on the Canon 24-105mm). Final rating is 8.4/10. The conclusion is here.