It has one pitfall which consistently stops me from using it though : poor support with newer releases of Rails, usually due to the Active Record stack not working well with the AR JDBC adapter.
I know some work was being done on a JRuby version of the standard pg gem (without the need for JDBC) which would be fantastic if it was completed and working.
Disclaimer: I now work on JRuby full time. But prior to this I ran Mogotest and switched to Sequel successfully on that project.
Anyone married to the MRI ecosystem because of dependencies on modules with no JVM equivalent may have problems, but new projects usually have no such issues.
When I last experimented with creating a standard Rails 4.2 app with the AR JDBC adapter and a database model a few weeks ago, it still didn't work. I haven't looked at things since then.
The workaround some people will suggest is "replace ActiveRecord with Sequel" but for existing projects a migration to a less standard Rails stack may not be viable.
I agree the JRuby team is doing a great job - however Rails is a big use case for any Ruby implementation, so my comment was not intended as a criticism, more in the line of a bug report and something it would be nice to have working now the new JRuby release is out the door.