Had a problem that I'd typically use Ruby for but needed to shed the GIL. JRuby worked beautifully. The only gotcha was I needed to use the JDBC driver for Sequel instead of the usual drivers. Obvious in hindsight but not so much with zero JVM development experience. I'd use it again.
> TruffleRuby is progressing fast but is currently probably not ready for you to try running your full Ruby application on. However it is ready for experimentation and curious end-users to try on their gems and smaller applications.
> TruffleRuby runs Rails, and passes the majority of the Rails test suite. But it is missing support for Nokogiri and ActiveRecord database drivers which makes it not practical to run real Rails applications at the moment.
Emphasis mine, but that puts it firmly in the "fun toy" category rather than a viable technology for anything mission critical.
The only problems I wouldn't go to Ruby for are CPU intensive ones or ones where I need parallel threads (which you already have Java for)