For those that cannot or do not want to move from Eclipse to IDEA or NetBeans, here are a few tips for getting by with the plugin for now:

1) The largest issue is a memory leak. I posted to the groovy user list about it over a year ago. (My explanation of the problem in that post was slightly incorrect. The problem will occur in any project with a lot of classes on the class path, not just one that includes another project.) I think the issue has gotten better in some circumstances, but it still makes it impossible for me to easily integrate groovy classes into my major projects.
Work Around: I have a small “GroovySandbox” project that has the class directory and key required libraries from my main project on its classpath. (Do not include the main project as a dependent project because you will have the exact same memory problem.) I write new scripts and classes in this project and then when they are ready, I copy them into the main project. The memory issue still has an affect, but I can work a full day on groovy files without having to restart eclipse rather than having to restart every 20-30 minutes.

2) Whenever I modify the class path of a project that includes groovy classes (thus triggering a clean build by Eclipse), the groovy class files are wiped with everything else and are not regenerated. I have to go in and touch each groovy class to get it to compile.
Work Around: I now have enough groovy classes that this isn’t feasible, so I created an ant target that compiles only the groovy classes and I trigger it from within Eclipse when necessary. Eclipse detects the newly generated class files and the remaining java classes I have that are dependent on the groovy classes finish compiling. (Why do I have to mix groovy and java so much? See #1, above and #3, below. Unless I want to take advantage of groovy syntax features a lot, I stick with Java.)

3. When I open a groovy class in the editor, it often doesn’t populate the Outline view. Makes it hard to navigate the file.
Work Around: Touch the file, or add-and-delete a space. This was a pain when using CVS for our SCM since the files were tagged as modified, but since switching to Subversion there hasn’t been a problem. I guess Subversion uses something beyond file dates to track mods.

4. When debugging, if I try to step into a class (java or groovy) that is included from another project (which I do frequently since nearly all classes are in the main project due to #1, above), I get an error window saying that the class is part of the Groovy Libraries plugin path and that the path is not modifiable. The screen doesn’t give you a button to modify the source lookup path for the directory/jar. (This one is relatively new. It started happening in some update around the beginning of the year.)
Work Around: Go into the project properties->Java Build Path->Libraries and manually set the “Source Attachment” property for the offending directory/jar. The next time you start the debugger, it will let you step in without a problem.