It works great for me and has a good mobile app. Gotta love the Swiss for privacy.

Recipe I used to create a totally clean fully functional email using TorBrowser was the following.

1.) Open Tor Browser... make sure its working.
2.) Go to hide my ass dot com.  Create a temporary anonymous receive only email.
3.) Go to protonmail dot com.  Create a new account.  On the "verify you are a human" step, use the anonymous receive-only email created in step 2 to receive the verification code.  Activate account and enjoy.

Thanks to Irresistibly Ivy for turning me on to it.  Been looking for this for a while.  As I tech professional, the methodology of two passwords and client side decryption is sound -- can't vouch for the implementation or people behind it though.