I can share much of the thoughts regarding RubyMine (a Ruby IDE by JetBrains). While it is not the best text editor out there (I used to use Emacs for Ruby stuff), the IDE features are great and (for me) outweight some of the niceties that Emacs or Your Favourite Text Editor has. Symbol navigation works across libraries and the graphical debugger is great. Autocompletion could be better, but I understand how the dynamic nature of Ruby (and especially Rails, which uses a lot of metaprogramming magic to do its thing) makes it sometimes hard to do 100% accurately.

(I'm not affiliated with JetBrains, just a happy customer)