For my copy, I have to set to +20 for micro AF adjust. This is the max setting. But luckily, at that setting, AF is spot on. But I am still going to bring it in to Sigma SC to get it adjusted FOC.
For D90, there is no micro AF adjustment feature, so die die have to bring it in to get it adjusted.
So my advice for Sigma, it is a good lens, but don't buy grey market.
Also this 24mm focal length (35mm FOV), you need to know how to use it to get the effect you want. Not easy to use lor. Moving in close to get the shot, you will be rewarded with very dynamic pictures.