Particularly for Java, where there's a lot of boilerplate that can be autogenerated IDEs are very useful. IDEs are great for method signatures and completion, as well as integrated debugging and unit tests (1 click run the tests again, add in your breakpoint and check your variable).
Some languages lend themselves to IDEs easier than others. For Ruby I use textmate, for Java I use eclipse. For Perl I use vi, and when I did .NET I used Visual Studios.
In the case of Java and Eclipse, and our million LOC codebase, the IDE provides a breathtaking boost in productivity. I find the main benefit to be the realtime compiler which catches compile-time errors, followed by ease of navigation through the codebase, and the remote debugger and code inspection.
I often think that not having this tool would be like what primates' self-awareness must be like: a glimmer of insight, but even the simplest thought requiring impossible effort, as in a thick fog.
Some languages lend themselves to IDEs easier than others. For Ruby I use textmate, for Java I use eclipse. For Perl I use vi, and when I did .NET I used Visual Studios.