I just bought this lens and got mixed result.
I found there is slight front focussing issue on 70mm and back focussing on 24-35mm.
It seems less issue on 50mm.
I will bring the lens to Sigma for calibration on Monday next week.
In reference to your post, I faced the opposite issue as you, back focused on 50-70mm and front focused on 24-35.
Just got my lens back from calibration and they seem to have done a good job calibrating it. It seems fine when i test it around the house, though images still seem a wee bit soft, but that could be due to my old D70. Will have to shoot outdoors to really test it out.
Overall, it's a really good value for money lens. The extra 1k for the Nikon just isn't worth it.