I just wish we could use an unlimited number of aliases in our own domain, it doesn't make sense to me otherwise..
They do have some interesting features, such as mailbox encryption as well as calendar/contacts encryption. It's client-side encryption, though it's in the browser.
An alternative to mailbox.org is mailfence.com.
What is a "normal user"? Previously, FastMail customers could get a Family plan that supported custom domains for as low as $10/user/year. New customers who require custom domains must now pay at least $50/user/year. A 400% price increase is a fiasco.