*The IDE understands well the relation between the files, you can easily jump between views and actions with a single click ( or a shortcut ).
*Finding the usages of a certain method, especially when you are planning to refactor a method name or something like that, the IDE gives you a whole list where a certain method was used or called.
* Fuzzy finding, either by file name or by method name.
* Nice integration withe Rspec, with the ability to watch and auto test when ever you change any piece of code.
* Plugins, most of the stuff you can think of has a plugin, some of the plugins I've used are the IdeaVim for vim mapping, editor config support, markdown support, and much more.
* Translation keys are substituted in line in your code, you can easily tell what a certain line in a view is gonna say without trying to look it up in the yaml file.
* key mapping for all common IDEs are available, so if you're coming from eclipse or netbeans, you don't need to learn new mappings, just tell rubymine you're used to what and it will use the exact same mappings for all the common tasks.
* The autofix menu, very useful for common tasks, like fixing typos, or some syntax errors.
* Nice integration with linux desktops, it generates the .desktop file and the bash script if you want to start it from the terminal.
As a Vim lover I've missed some of the vim features, like vim surround, also the vim mapping conflicts with a lot of the IDE shortcuts, so you'll need to access those from the tools menu, or something like that.
Since it's built on java so it eats up some RAM and takes some time to start up.
RubyMine already provides a 30 days trial, you could take it for a test drive before you decide if it's worth your money or not
Also when RubyMine is creating a new version, it releases the testing builds for free to test all the new features, so this could give you more time to use RubyMine without the need to actually buy it.
I've used this product for my development tasks, I've used at a lot at the beginning and it was really useful, and it honestly improved my performance, but then I've decided to move to vim to be able to use some plugins, but I still miss some nice features that were available in RubyMine.