Pretty good actually. Possibly even best in the market. http://www.infoq.com/news/2007/08/intellij7m2 is an article about just this. Also Grails project leader Graeme Rocher couldn't say enough nice things about IDEA during his presentations at NFJS London. So I'm guessing he also thinks it's pretty good for developing in Groovy.
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The Groovy plugin which is currently in beta provides quite powerful support for Groovy and Grails development - we aren't aware of any competitors that provide similarly advanced functionality.