So it looks like the Basecamp folks are ready to head down the home stretch of personal domains for Hey for You. They have still be very circumspect about whether this will cost more or not, but I have in the intervening time had to stop and thinking whether i want it.
I own a custom domain, not due to personal branding or a business, but just for consistent online identity, and more importantly, email portability. When I tired of GMail, and moved to Hey, I didn't need to change much. I just redirected mail to my pictor.me domain to go to Hey, job done.
Now this reasoning stands, but custom domains on Hey, done right, will carry one disadvantage. I forward my email to Hey, AND to my iCloud account. In this way, I get a redundant back up of my email in the event that Hey for some reason goes offline (or...to be frank, if I elect in the future to stop paying for it). I chose iCloud simply because I also trust Apple's approach to privacy, and it's a functional backup dumping ground of email even if I never go there.
Custom domains means I would no longer be forwarding email. Hey would become the actual manager of email traffic of my domain. That means (I think?), it would only go to Hey. My registrar would no longer be getting that email, and forwarding to my 2 addresses. I keep the portability of my domain, but I lose the redundant backup.
Do I care? I'm not sure. Maybe not (I've never needed or looked at that backup once), and being able to reply to people with my proper domain, not proxied out through outlook.com which is what I use as my SMTP server to maintain the 'from' address I want, well, there is something appealing to that.