I spend about half my time developing in Ruby and half in Java. For Java development, in my opinion nothing is as good as IntelliJ – I love it! I am still in the exploring mode when it comes to Ruby development. When using OS X, I sometimes use TextMate and I like TextPad on Windows – both inexpensive and very useful. That said, when I am dealing with larger programs, I find Eclipse and both the Ruby and the Rails plugins to definitely be ‘good enough’.
Actually, as a ‘good enough’ platform to build other platforms, Eclipse is the ‘best of breed’
In the last few years, I have used Eclipse (with suitable plugins) for Ruby, Python, C, C++, and Prolog. Eclipse is also very good for Java, but not the best.
If there is one thing that I like best about Eclipse it is probably the subversion and CVS support.