This functionality is welcome. What was previously managed with multiple browser accounts can now be managed with only one (provided I use the same add-ons).
A few things that would make its initial and possibly regular use more convenient in my case, mainly different ways to sort websites into a container and make it defualt.
--ability to import bookmarks and bookmark folders so that they defualt to a specific container.
--ablity to sort "current tab" and "all open tabs" into a specific container when clicking on the add-on icon
--had minor issues with [are you sure you want to open this site in this container by defualt] message where it was sometimes unresponsive (I added an entire bookmark folder to a container and opened all at once) I mostly had to wait and reload before confirming.
--The ability to generally avoid/disable the [do you want this to defualt to this container] without individually using the [rmember for this site] setting might be welcome.
Consideration: if I'm already in a particular container and link to a site that would normally use a different container I would most likely not want it to instead remain in the current container or have me decide, unless in the defualt container. If the [remember for this site] option is not selected the message does serve that purpose but it's not useful when opening bookmarks. So implementation may have to be complicated or have to allow for various rules that are dependent on the current container.