I've been using IntelliJ IDEA (12) with the Ruby plugin for some time now, to develop a hybrid Java/Ruby application. In this case, we're using Rails for the front-end, and Java code for communication with our back-end servers (we already had the communication layer built in Java before we started using Ruby).
Some things I find would be helpful:
- Dual-language debugging: It would be nice to be able to debug the Ruby code (set breakpoints, etc) while in the Java debugger, or view Java data structures while in the Ruby debugger.
- Better project / build support: I build my project with custom Rake tasks, but it took me a fair amount of work to get it to all come together
If some of this is available with IntelliJ IDEA 13.1 and the latest Ruby plugin, then I will get my boss to upgrade us.