Coming from a background of developing in languages such as Java, one of the things that surprised me the most about the Ruby and Rails community, was the common practice of not using an Integrated Development Environment. Most of the members of the community, including the most relevant, were comfortable with just a programmer's editor.
At first I thought it was because, Ruby being a dynamic language, using a full IDE might be an overkill. But then I thought of the PHP community, in which several IDEs are popular, with PHP also being a dynamic language. So I still had to guess why using an IDE was not a common practice within the Ruby on Rails world.
Nowadays, there is a growing list of IDEs with support for Ruby on Rails, but two years ago the options were really scarce. Back then, I chose to use RadRails because it worked on top of the Eclipse IDE—which was the tool I was already using for other programming languages—and because it was the only free, open source, and portable option.
Truth is, the first version of RadRails I used was very promising, but still a bit too basic. It featured just a few specialized tools, Ruby syntax colorization, and a slow and faulty code-assistance. As a result, the difference between RadRails and a good programmer's editor was not really significant. However, as Ruby on Rails gained popularity, RadRails was vastly improved, and a lot of new features were added.
At the same time, several other IDEs started to provide support for Ruby too. Today, even if many Ruby on Rails developers still don't use an IDE, a growing number of them already.
During these two years, I've been developing projects almost exclusively with Ruby on Rails; and I developed all of them using RadRails. Of course I have been keeping an eye on every new IDE with Ruby support, just to see if there were any reasons for changing, but I still didn't find any.