Fastmail had a UI change a year ago, where they went all "rounded corner" and created unique rows for every message that weren't so easily sorted with Stylus CSS. I emailed them about the changes, and a few weeks later the messages were listed in a way that I could easily apply my own stylesheet again. They also acknowledged on their subreddit the UI change was not incredibly popular (even the CEO was "not enjoying" it) and they took some feedback and provided a few better UX customizations in the options.

At one point, Fastmail had an error parsing Unicode in misconfigured iCal files (Google Calendar showed events correctly) but after a short back-and-forth, they fixed it within a week.

Now, Google Calendar has had a problem with my organization's iCal files for the past four weeks. Submitting a bug report via the help community is very opaque and unhelpful (a la "Have you tried turning if off and on again?"). Fastmail loads the iCal correctly. I have no clue if Google is aware or when they'll ever fix it.

One huge productivity advantage to FM over Gmail? Sort by sender name. (This makes it so easy to bulk apply changes)

Setting up own domain has been straightforward. (It's a bit of work ensuring DNS records have DMARC/DKIM/SPF, but it's all in the FM checklist/documentation.) Setting up Python scripts to auto email me using app passwords has been straightforward. Creating aliases and throwaways is straightforward.

I have not regretted paying for email; specifically, Fastmail.