To tell me to "just use" it seems a bit glib considering that it's apparently nowhere being ready for production (the current code even has failng tests), let alone available as a swap-in replacement for Ruby.
That said: It looks like a really interesting project. I do wish they would phrase the project description as a "Ruby VM" instead of a separate programming language, through. There should not be any need to fork the entire language just to provide better performance.
https://github.com/manastech/crystal