There's been a couple short downtimes, always documented/well-communicated on their Twitter feed. I think I've only ever been able to not get into my email once... and it was fixed in a few minutes.

And outside of general service reliability, their support team is top notch. I can't say enough about being able to file a ticket on a problem with my email service and hearing back from a real person about it.

Also: https://www.fastmailstatus.com/