I've owned the 10-20 vari aperture, worked fine on XTi, don't know it would be on higher MP cameras.
Currently own the 11-16 f2.8. I need the f2.8 for video. You will need to be confident using the defringe tool on occassion in PS, apart from that its a great lens, low distortions for FL, sharp etc.
I used to own the 12-24 Sigma, which gave me great images, just it was a pain to filter, rear gelatin only, the same will be true of the 8-16, so if you have designs on using heavy ND's or polarisers then it maybe isn't the best lens.