I'm currently still in the free-tier. But I'd be perfectly happy to pay for the product. It's super easy to use, and the happy path of the cli tool is well designed.
The various configs for the fly.toml file could be better documented, as could some of the more advanced cli commands like sftp and ssh into your machine.
I think fly really shines when you design your app intending to run it close to users. If you just want to host an app, there are other hosting products. But fly is particularly good at hosting and scaling into different regions and presenting those regions as a single app. Trying to do the same on AWS is messy.
It's also nice to see them investing in the database problems you get when trying to run at the edge. (Like the db being in one region, which defeats some of the value of running close to your users).