The price is a deal-breaker for me. It's borderline insulting, having to pay a subscription fee for an already-paid email service. Were it a value-add, it'd be compelling.

-- edit --

Turns out it's not made by Fastmail, it just uses "Fastmail's standard" of JMAP. I'm still not going to pay a monthly subscription for it; a one-time fee would be much more palatable IMO.