I use IntelliJ with the ruby plugin (which is identical to rubymine). Very nice, but I often resort to the command line for running git commands. The main advantages over "nothing" are syntax highlighting, code completion, being able to search all the ruby libraries (for your RVM). Downsides are the debugger works "most of the time" and bundler seems often munged up (it works, but reports errors).
I've also used <a href="http://www.aptana.com/products/studio3>RadRails</a&#...; which is also pretty good. Frankly, I like jetbrains products because the support is awesome and they just seem to be better at guessing my intent (with intellisense), but I wouldn't kick RadRails out of bed in the morning ;)
I've tried to use every IDE out there -- from Aptana RadRails to NetBeans, and RubyMine. After lots of frustration and headache, I've gone back to using the command line and TextMate. I actually think that TextMate while not "Integrated" with every aspect of development, has the two key features you really need -- code completion and syntax highlighting.
This isn't a code-maniac purist perspective either -- I really wanted to use an IDE and tried hard, but between the setup/dependences of the application, or the tricky integration with git, or the lack of visual CSS support, then something else (usually version incompatibility) would end up biting me every time I tried.
To answer your actual questions:
Maybe, maybe, yes, YES.
To answer the implied question: "Will an IDE help me develop a Rails app more easily?" IMO, probably not.
I use Vim which definitely has a learning curve. The advantage of Vim is it's available in most unix environments and it's cheap.
If you're ever on a prod server and having to hack some code (obviously I don't do this) then your Vim skills will help, it's usually pre-installed or at least Vi will be there.
The thing with rails is you don't need an awful lot of help from the IDE. The cycle is usually...
1. Edit some code
2. Refresh the browser, look at the results.
3. goto 1.
Unit tests and the console I run from the command line.
I have not, but someone I know has. It's called RubyMine (http://www.jetbrains.com/ruby/). For an individual developer account, it'll set you back by $69.
I've also used <a href="http://www.aptana.com/products/studio3>RadRails</a&#...; which is also pretty good. Frankly, I like jetbrains products because the support is awesome and they just seem to be better at guessing my intent (with intellisense), but I wouldn't kick RadRails out of bed in the morning ;)