Yes, we use it and are happy with the service. Not so much because of its security / encryption features, but because it’s a Belgian based, independent email provider and I applaud that. I just moved my family there from Gandi after they screwed their users with price hikes and false promises. For context, I am Belgian and have met the founder. They have a nice mission. I still self host my own email on my own mail server for 20+ years. But if I weren’t, I would probably use Mailfence.
What I also appreciate about Mailfence is that they stick to using standard protocols. Protonmail needs an intermediary layer to provide IMAP services for instance if I recall correctly. Perhaps they (Protonmail) do this to arguably offer better encryption services, but to me email is not very secure to begin with, no matter what you do. You’ll always have your messages pass through other people’s servers and systems and not all your recipients will be comfortable with PGP or s/MIME. I love email as an OG federated service though, despite what some may consider flaws in this modern age. It’s such a core and empowering technology of the internet!