I use ProtonMail and overall it works pretty good and the value is great. The product is focused on security first so I think user experience suffers a bit. It's works fine but if you previously used a mail client like Spark on your phone you'll definitely miss it.

The mobile app is okay, it's good enough for reading and sending short replies. The web interface is alright. I like to use the bridge application from ProtonMail on my desktop and use a mail client like Thunderbird or Apple Mail.