Almost useless. I regularly have to switch server because the connection just stops working, and now I'm getting the error "The proxy moz-proxy://https-us-washingtondc.privateinternetaccess.com:80 is requesting a username and password. The site says: “Private Internet Access”"

I'll be switching VPN providers when my subscription ends

Update: 8 months later I'm still having to switch servers, and reinstall the add-on every week or so.