It is a must have extension. I primary use it during development where I need different states of cookies and logins.

The only thing I'm missing is to be able to open a URL in a specific container e.g. by specifying a protocol prefix in a href (or whatever data-x-target="{container name}").
I can not assign a full domain as I need to view the same domain in different configurations, but I'd like to access them by just clicking on my "bookmark" link - My site (Beta-2), My site (Alpha).. etc...