The Sigma 24-70mm F2.8 DG OS HSM Art is, like the competing model from Tamron, an attempt to deliver a wide aperture standard zoom for full-frame SLRs at a reasonable price and with image stabilization. That's beyond ambitious.
Consistent performance through zoom range; Optical stabilization; Solid build; Dust and splash protection; Available for multiple systems; Attractive price
Weak edge performance at f/2.8; Dimmed corners; Visible chromatic aberration; Some distortion