I've been using Hey since it was released. It's fine, but if I was going to make the switch today instead, I'd choose Fastmail or Proton instead. I use Fastmail for my business and it's been quite good. Proton conceptually feels like a better choice, but users with Proton-domain emails cause a ton of headaches (fraud, etc.) at work, so I'd probably give them a pass unless you use a custom domain.