The addon works quite good but it also has some flaws:
If you change passwords on other devices or even using the normal Psono website in the same browser, the addon does not recognize these remote changes automatically which is a bit annoying. You first have to open the database using the addon again and then you also have to refresh the login page too where you want to use the freshly created or modified entry. It would be much nicer if there were some kind of refresh button or an automatic cronjob in the background which polls recent updates on the remote side automatically.

Furthermore sometimes the autocomplete window of Firefox itself overlaps the one of Psono which is also annoying because you can not click directly on the entry of Psono.