I find their UI to be simple and clear. I enjoy the notes/calendar features that come with the email. Their mobile app works fine on my android phone, and the push notifications from the Fastmail calendar are relatively unobtrusive.
Only downside so far was I had to cancel my NYTimes sub (honestly probably a plus) because it doesn't recognize the @fastmail.com domain as a valid email address. It's the only service I've had that problem with, including the important ones like banking, etc.
I'd advise anyone to use their own domain, but those aliases can be a godsend.
I was unable to sign up for an Etsy account with Fastmail. They only acknowledge the majors. But I was able to sign up with gmail and then promptly change it to fastmail. Really sophisticated opsec on their part.
I used to not like Outlook at work, but when I worked for a startup they used GSuites and the Google mail experience is so bad I honestly couldn't believe that anyone would be using it. Threads are mixed together when multiple people reply its impossible to read content anymore and they do also change the formatting of content which puzzled me the most.
Maybe for people that only ever used Gmail it's okay, because they are used to its weirdness and have never seen a more structured and visually appealing way to read email.
Maybe the lesson here is that if you use one of them for years, then the other seems really bad.