Crystal is fast becoming pretty useful. Especially with the recent news about Parallelism(not concurrency) added to the language.
Before I started Elixir I was looking long and deep at Crystal.
I also looked long and deep at my current code bases in Rails and realized I could not run them with Crystal without a ton of rewrites — especially 3 years ago.
So we made the jump to Elixir — because there was already a lot available on the BEAM that Crystal did not have as yet.
If you were starting something new and you were a Ruby shop, Crystal would make sense at this point.