Andy Yen, the CEO of Proton (Mail, Drive, VPN, Pass...) answered a lot of the questions you, the community, asked, in an interview that covers basically everything! He discusses security, privacy, the origins of Proton, how they operate, Linux support, future projects, products and features, quantum...
They have some shady (or at least questionable) enough actions in their past, some even covered by mainstream media, that made me dismiss them as an option. I went with the German hosted mailbox.org instead. Swiss law (where proton is hosted) is actually quite a bit less protective of privacy than EU/German law, or maybe just protected in other ways. The international reputation of privacy protecting character of Swiss law seems to be outdated?
Just to be clear, I can't remember exactly what the specific events were that caused me to reconsider back when I switched years ago. When I just did some quick (!) searches just now, I found statements that they would only record ip addresses in "extreme criminal cases", but examples include cases of trespassing and property damage. Not exactly child molesters and serial killers (example source). I also understand that the (Swiss) laws relevant to them probably forced them to, but at the very least that seems dishonest or misleading advertising.
I pay for the subscription and if I didn't I do not mind free services asking for money. If you are giving me a free service and not data mining me then you have every right to nag me.
Gmail litterally puts ads in your inbox that look like emails so I doubt any service is gonna be worse than that.
They mentioned that they bought premium, so they won't be seeing ads. Edit: Multiple people below are telling me that purchasing premium does not remove all ads, so that's actually quite annoying.
As an aside, I really don't get the logic of using a service like e-mail and just expecting it to be free - it's very bizarre to me.
If it's something important to you (as I assume e-mail is), pay for it - or stop whining about ads/nagging pop-ups.
And if you can't pay for it, download Dovecot and try doing it yourself - it's a fun experience, and will make you appreciate the work done by email providers. Spoiler alert: it's a massive pain in the butt.