The extension looks the part but has usability issues that need managing manually.

The main usability issue is that if you're connected to a VPN server and the computer goes into sleep mode then later resumes, the connection isn't refreshed/handled properly, effectively blocking traffic until you manually disconnect/reconnect. Occasionally this issues causes the GUI to become unresponsive too, requiring the extension itself to be restarted.

There's also no feedback given on the connection quality/speed of the server you're currently connected to. If it starts to feel slow, it requires manually moving to another server; there's no automatic transfer to a better equivalent server within the same country.

Not a negative mark on the extension per se - but it's worth noting that there's a growing list of websites that will detect you using a VPN (likely through matching your IP against IPs known to belong to Proton/etc.), making VPN usage for 'general' anonymous browsing more and more frustrating.

Proton VPN itself works well and provides lots of protections and connection configuration; it's just the extension that has a few issues really.