Until recently I dismissed JRuby, but then I needed to run some stress testing code against an Oracle DB. With MRI Ruby and the OCI8 library, the Ruby overhead stopped me being able to push the database hard enough. I switched to JRuby and JDBC and the results were amazing, so I am converted. The only niggle I have is the startup overhead, which I know is a JVM thing. Not a problem for my benchmarking, but a little bit of an annoyance for a command line ruby tool I use.
As an aside, I did run across a bug, not performance related, but a case around popen where JRuby doesn't behave the same as MRI - I raised a ticket (JRUBY-6225) but it doesn't seem to be getting any traction. I understand there are probably plenty of tickets to be prioritized, but this one is a bit of a show stopper for me. If anyone from the JRuby team is see this and has time to have a look at it I would be grateful.