JRuby's a pretty solid implementation. It's generally faster than MRI 1.8.x, and has the benefit of quality threading. I think Scala is probably ahead, becaus

JRuby's a pretty solid implementation. It's generally faster than MRI 1.8.x, and has the benefit of quality threading. I think Scala is probably ahead, because Scala has the benefit of being-more Java-like than JRuby, and is therefore easier to implement. For my serious apps, I typically write the front end in JRuby on Rails, and migrate code back into Java as it stabilizes and/or needs more performance. Works great for me!