They're a nice feature, although after a long while of trying, I ended up using just the Facebook container. The main container extension felt like too much work in regular browsing, and the tracking protection seems otherwise adequate when combined with uBlock.

Sadly, Mozilla also seem to promote containers as an alternative to user profiles, while they're nowhere near as full-featured - sharing saved logins and bookmarks between my "personal" and "work" containers is almost never desireable. Managing and switching profiles, on the other hand, is virtually unchanged since the Netscape Communicator days.