I recently switched from Gmail to Fastmail as well, and I like it so far.

For me, scanning my email to show me ads didn't really bother me that much. But the thing that I really like with Fastmail is that my outbound email alias works the way I want.

The university I graduated from gives us email aliases for life, and I prefer to use my university alias instead of my Gmail address. It obviously works fine when I receive mail, but when I send mail from Gmail it would say "from me@gmail.com on behalf of me@jhu.edu" (or whatever). And so for a lot of people, when they would add me to their address books they'd end up adding my gmail address because that's how it was presented to them.

With Fastmail everyone just sees my jhu.edu address as I prefer.