I picked up a used copy of this lens at a good price and when I got a chance I broke out FoCal Plus to calibrate the MFA on my 7D and 50D.
Testing with the 7D indicated it was front focusing quite a bit, averaged out at +13 on the wide and long ends. Wasn't news to me since I had read lots of people experiencing front focus including the review at The Digital Picture (in his case, 3 out of 3 copies front focused). I was surprised that there wasn't any difference between MFA values at the wide and long ends, all the zooms I have tested so far had an average gap of +/- 10 between ends on this body.
When I put it on the 50D however, it started to get ugly. FoCal came back with recommended settings of +18 on the wide end and +22 on the long end. The latter obviously not being possible with maximum MFA adjustment available being +20. I've sent off an inquiry to Gentec in Canada to see how much it would cost to calibrate the lens.
I did send the seller a note to ask if he had focusing issues (my fault for not asking when I bought it) and he said it was spot on every time. Maybe he was just lucky with his setup as he never indicated doing any kind of MFA, but I find it a bit hard to believe.
Anyways, if anyone else has experienced focusing issues with this lens I'd like your input. Thanks!