Any good calibration system will work well. I use the i1 Display Pro which works fine on my iMac and rMBP.
It was claimed that the new iMac's would be factory calibrated to an international standard. Now no mention as to what that is. I assume it means normal profile calibration. However no mention to brightness level, which is usually a problem with most displays ( not just Mac's) is they are too bright. So additional calibration is required
especially to tame over bight displays