I use Fastmail for my personal account. I use gmail at work.
Fastmail's UI is a ton more user-centric. It just feels more thoughtful to me.
Fastmail treats keyboard shortcuts as first class citizens, so you can run it almost completely mouse free.
Spam filtering is on par with gmail, if not better.
Aliases and throwaway accounts are easy to use if you have your own domain. I don't know how gmail handles that.
When you click a link from fastmail it doesn't pass through a fastmail server as gmail does, but instead goes directly to the source.
The calendar is decent but it's a hassle to import .ics files. It's readable and searchable though. If you attach external calendars then there's some delay when you change something on the external calendar. That can be annoying.
The Contacts manager is OK. It's definitely not special but that also means it's not over-engineered.
They have some file storage and notes capabilities which I don't use, and honestly I worry that it's the beginning of adding features for features' sake, but if that continues I can always keep fastmail and move back to Thunderbird or whatever, although that would suck a little bit.
The android app seems good, but I came from K9 mail which wasn't the most polished. I don't use gmail on my phone so I can't compare, but to me Fastmail's app proves the adage that the better a tool is, the less you're aware you're even using it. It just does its job and stays out of the way, which is cool.
I guess adding new calendar entries on a phone can be annoying. Like you tab the box to add a contact and the dropdown menu is obscured by the edge of the screen, stuff like that.