I use IntelliJ 7 and its groovy plugin is excellent. It's hardly a plugin - it's distributed as one, but it's maintained very actively by JetBrains themselves. With the latest plugin running the groovy editing support is almost on par with their java editor, and that is quite an achievement.
-Tim Fennell<br /><i><a href="http://www.stripesframework.org/" target="_blank" rel="nofollow">Stripes</a> - Because web development should just be easier.</i><br /><i><a href="http://www.pragprog.com/titles/fdstr/stripes" target="_blank" rel="nofollow"> Stripes: ...and Java web development is fun again</a> - An awesome book about Stripes.</i>
I've worked a little with the Eclipse plugin, and I can tell you from experience that it's nowhere near as good as the IntelliJ plugin. Working with Eclipse, I had trouble: (a) importing a Groovy project, (b) with code assist - sometimes it just doesn't show up and throws a NPE, (c) with running scripts - sometimes they just don't run and Eclipse shows an error message. I'm hoping that plugin gets better, as Eclipse is my default IDE. I've heard that NetBeans is even worse than Eclipse for Groovy support, and I'm only using IntelliJ at work.
Here is an article discussing IDEs and Groovy. [ April 08, 2008: Message edited by: Josh Brown ]