Among the problems, and they are not mere annoyances, are reduced battery life, application crashes, Wi-Fi connectivity problems and complete lock-up that can only be remedied by a battery pull. That does not sound like much of an update.
For what it is worth, it looks like Samsung has pulled the update rollout for the time being while these issues are looked into. The update to the Samsung Galaxy Note II may also be placed on hold since it shares similar underpinnings with the Galaxy S III.
Samsung is not always able to stick to its timeline when it comes to updates, but at least it puts forth the effort. Once these issues are ironed out, we hope performance will improve and those that are (im)patiently waiting will see your updates soon.
In the meantime, if you have an official update for the SGSIII, or if you installed the leaked update, let us know if you have been encountering any of the above noted performance issues.