Eclipse, starting with GGTS from SpringSource used to be the best editor for working with grails projects. During the switch to Grails3 though things have changed. The GGTS Eclipse plugin is no longer being maintained.
All this was not such a problem the first months or year even. After that Eclipse, Java, Groovy and Grails kept upgrading, but the plugin did not. This lead to more and more tiny problems and failing features. The last few months the experience of using Eclipse degraded so much that effectively using it became virtually impossible.
This has lead me to use IntelliJ (Ultimate), and getting a license for it. The experience is great. I can work on grails like I used to including auto suggest, refactoring, etc.
The downside, which kept me a long while from switching, is that IntelliJ is proprietary and this does not do much good for the open source community.
For Grails3 projects, using Eclipse is fine since most of the features will be offered by either the Groovy Eclipse plugin and the Gradle Eclipse plugin, both of which are supported really well.
For Grails2 projects, Eclipse is no longer.