Crystal is beautiful. I wish I had a use case for it.
It would be a totally different thing if it could run Rails, which is one of the things I do in my job. The speedup would probably justify using it, especially when running tests on my laptop. My customers are perfectly happy with the performance of Rails on their servers. Same with the ones using Python with Django.
At https://heiioncall.com/ we combine the two and use Rails for the human-facing parts, and Crystal for the machine-facing parts (inbound API server calls, and outbound HTTP probes). For us, the number of machine-facing requests per second is so many orders of magnitude higher than human-facing requests that Crystal's performance and lower footprint is valuable there. While on the other hand, the conveniences of Ruby on Rails are still great for a conventional human-facing web and mobile application.