This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
You'll currently get the best Groovy Grails support from an IDE in IntelliJ IDEA. I've have heard that good things are happening with Eclipse and Netbeans, but IDEA is the current frontrunner.
On another note, it is very hard for an IDE to give you the support your are used to with Java when you're using a dynamic languages. There are just so many possibilities about what you are meaning when you type, the IDE almost has to be running the application to know what you are referencing.