I find Rubymine very useful. Some of it is just wrappers to shell commands, but other parts have syntax highlighting and suggestions and catching syntax errors. I like the git VCS integration and the debugging. I program Rails with it and really find it useful.
I agree and use both RubyMine and TextMate, usually separately, but when I am coding in Ruby (usually Rails work) I sometimes like to keep RubyMine and TextMate open on the same project because it is so fast to zip around to look at different files in TextMate. (I do the same sometimes for Java, keeping both IntelliJ and TextMate open.)
It may seem perverse keeping two editing systems open at once, but it works for me. I also even more perversely often keep TextMate open when I am working in Common Lisp or Clojure with Emacs.