I just tried upgrading one of my live sites to 6.6.0
When I tried to open the front page in Cornerstone, it was as if all the cornerstone data was erased. Other pages seemed unaffected. I had done the migrate HTML process before I tried to open the front page in Cornerstone.
Since it was a live site I quickly restored from a backup and all is ok again.
But if you want to investigate want went wrong I can create a clone of the site for you to play with - please let me know.
Steve