This is extremely useful both to protect my privacy and keep environments separate. There are two essential features missing, in my opinion, though.
First, the option to reorder the categories. As it is now, if I want my tabs to appear in a different order when I click on "Sort tabs", I have to manually delete the containers and recreate them.
Second, for users who keep lots of tabs open (like me), it would be nice to have sessions only within one container, so that everything in the session will be automatically opened within it. At the same time, other tabs opened in different containers would stay stored so that, next time you open a default session, all types of tabs will be opened together.