This add-on is a good idea and I really want to use it. But there's a few things that keep it down for me
1) Lack of wildcard subdomain support is a killer. I read the issue on the subject and it seems there's privacy concerns with that. But the fact is that all the big sites you'd really want to confine to a container use subdomains constantly and often so it makes it frustratingly difficult to actually confine a site like google to its own container.
2) I could probably live with #1 if there was a way to tell the extension to open the current site in a specified container in the future. Right now it appears you have to copy the url, use the extension to make a tab in the desired container, revisit the site and then tell it to stay there. That's a whole lot of steps for something you're going to have to do all the time without wildcard subdomain support.
3) Once you're in a container, all navigation done from websites in that container remain in that container. So for instance, if I put google in a container and then click a link the search result, that link opens in the same container. This completely undermines the point for me, I can't actually use this to keep a site from tracking me unless I go put every single site on the internet in its own box manually. That's entirely too much effort, especially given the difficulty of actually putting a site in its own container (see #2)

I'm giving this 3 stars and will probably keep using it because it is still convenient as a multi-session tool. Going to the same site in multiple sessions is something I have to do for work a lot so it is handy for that. But I'm hoping it evolves because it could be quite a lot more if they made it easier to just completely containerize the sites of your choice.