Duncan F,
One of the guys at my work uses one for magazine cover shots etec and his si the Canon. It's a well-built lens that focusses really quickly but that's about as much good that I can say abotu it*–*vignetting and horrific abberations when it's wide open mean he's had piccies printed that really do look shocking. Not seen the Tokina one but a few of the other guys use the Sigma and it's noticeabley better, although not perfect. A good lot cheaper though.