From the mailing list discussion about Groovy vs. JRuby:

1) Have you actually used JRuby for any real world applications?
2) What is it best at? What is it worst at?
3) How well does it integrate with Java?
     (Groovy and Java are highly integrated in several different ways.
      At a minimum, Groovy classes can extend Groovy and Java classes
      and interfaces. Java classes can also extend Groovy classes and interfaces.
      The Groovy MOP protocol adds even more power to the integration).
4) How does JRuby resolve the conflict between Java and Ruby data types?
4a) Does it support coercion? Autoboxing and unboxing?
5) How faithful is it to the Ruby language?
6) Does it implement all of Ruby?
7) Can JRuby use Ruby libraries?
8) Is JRuby up-to-date with mainline Ruby development?
9) How fast is it?
10) How good is the tool support (IDEs and so on)?