Being an owner of IntelliJ Idea Ultimate edition, and having installed the IntelliJ Ruby plug-in, I'm forced to wonder how different that experience is from using the RubyMine IDE.
I'm guessing they share a lot of code & features. What are the major differences?
If I'm jumping into the world of Ruby, should I buy the RubyMine IDE or just continue using IntelliJ + Ruby plug-in?
The Ruby plugin and RubyMine are build from the same codebase, that's right.
Java/Ruby integration for JRuby code is only available in Ruby plugin for IntelliJ IDEA. But the plugin doesn't have RubyMotion support.
Also check this answer: If I have IDEA, do I need rubymine for ruby dev or will it do both?