Personal claim that not even they can open the files on its system. This is because they do not know your password which exists as a secret between you and the gem.
The downside is that if the password is lost it cannot be recovered. I set up personal and felt sure I had remembered the password. Alas. My gems disagreed and I lost the data.
I half suspect that my password was correct and it was the gems that forgot. What we gain in privacy we lose in convenience.