Most Belighters, as probably most Rails developers, use TextMate (the most hardcore ones use Vim, of course). Yet some were seduced by RubyMine. Here is a small comparison of the two tools, which will hopefully help you understand our choice (although we are expecting some hate mail).
RubyMine “understands” the Ruby/Rails syntax (along with JS, CSS, Haml, Coffee, …), which yields several advantages:
lots of shortcuts to easily navigate through your code (jump to method definition, to view, …)
syntax errors highlighting
easy refactoring (rename variable or method, extract method, …)
Quick access to documentation (Ctrl-j opens RDoc in a tooltip)
Integration with Git (file comparison), Bundler and other tools
Updated frequently (Rails 3.2 is supported as of the latest update)
RubyMine is expensive (the commercial license will set you back 142€, whereas TextMate is less than 45€)
As most IDEs, RubyMine loves your RAM
It should also be noted that, Ruby being dynamically typed, the “intelligence” of RubyMine is still limited compared to similar IDEs for statically typed languages such as Java.