I started using Proton Mail in 2016, back when they were called "ProtonMail" and email was the company's only offering. Their Calendar was actually built-in to ProtonMail at the time and wasn't a separate app. I switched to daily-driving this email provider in 2016 and switched to using their calendar app in 2019 from Google Calendar. I could not get their Proton Bridge app working very well on OS X back in 2019 and have been using the web app version instead ever since.
I do not use Proton's other services, not because I don't like them, but because I'd rather not depend on any one company for so many critical functions.
Overall it's been... okay. If they implemented the quality of life improvements (e.g. search bar for Android Proton Calendar events and Android Proton Mail contacts, ability to edit a shared calendar event, hide spam toggle from "All Mail" email view) people have been asking for years for the experience would be much better, but overall it's clearly not a deal-breaker for me since I'm still using it.
Between the bugs in their apps, the slow rollout of basic features, the cost being a little high for what you receive (especially if just paying for email), I think it's a tough sell if you have high expectations coming from Google services. However, if all you care for is basic private/secure functionality, then I think Proton is a fine choice.
