Sidekiq was recently ported over to crystal and the speedups and memory usage improvements are amazing. There's also an officially supported heroku-buildpack so it can be deployed easily.

But there's a lot of breaking changes in the language (0.18 was released a few days ago with a few breaking changes). I would not use it for a long term project (yet). But for some personal project or prototypes, for example which you'd quickly set using Ruby/Sinatra, I'd choose Crystal/Kemal.