Original answer (left so people's votes aren't misrepresented):
Current IDE status (as of Mar '09) for Grails dev:
- IntelliJ Idea still the best, but costly
- NetBeans 6.5 is MUCH better than 6.1, but released before v1.1, unsure what the 1.1 changes may have done to this.
- Eclipse is still far behind. However, SpringSource is a major player in Eclipse, and they now own GOne, the main developers of Groovy/Grails. This is supposed to have the effect of speeding Eclipse plug-in development, but no results so far.
It's now Dec '10, and things have changed (but not too much):
- IntelliJ Idea is still the best, but only marginally so, and expensive.
- Eclipse now has the STS (since Spring is a major developer), with a lot better Grails integration than before, especially for new projects. If you're using Mavenized Grails... you may still want to stick with IntelliJ.
- NetBeans 6.9.1 is out, but its Grails support is stagnant since 6.5. It's now the bottom of the pack.
And now it's Feb '15, almost 6 years after the original question. Much has changed, but much remains the same:
- IntelliJ is still the best, and by a growing margin. Grails 3 (due out next month) will actually allow development using the Community (Free) edition of IntelliJ
- GGTS (The Eclipse package created by VMWare) has never really caught up, and will only get further behind now, as Pivotal has ceased its sponsorship of the Groovy/Grails team, and will probably also no longer drive GGTS development.
- Seriously, does anyone still use NetBeans? Ok, I see they still support Grails, but I don't really have any idea how good that support is. Everyone I know... EVERYONE... uses IntelliJ.