Just moved our app (tourcandy.com) from Heroku to fly.io. The experience was not great.

The product is far less mature than Heroku and the pricing is confusing. For example, it took us 3 days to figure out how to run Redis properly (their integrated UpStash Redis has quite a few bugs. We ended up going with self-host Redis on fly.io). Besides, we have no idea on how the pricing would work when fly.io shows our app as one app but there are actually 3 processes running (web, Sidekiq and Redis).

The good part is it's seems much cheaper. And because it has more edge nodes to choose from, it's also much faster in our case.