Google is suggesting to use Eclipse
as editor. Google has written Plugins for Eclipse which makes it more
easy to use GWT. Also there are other plugins such as the GWTP plugin that automatically adds Model/View/Presenter based classes and dependencies. Please note that the GWT Designer can only be used when you are not using UiBinder and want to use code to create the GUI. GWT Designer currently isn't supporting UiBinder yet.
So if you look for a more advanced solution and one that will always be supported by Google it is recommended (also by me) to use Eclipse.
Netbeans is a very advanced really easy to use editor. I prefer to work with Netbeans as it is more intuitive then Eclipse. It is a petty that the GWT and AppEngine plugins are far not as mature as the Eclipse plugins. But for those who want to walk further with Netbeans I have written a basic tutorial how to use the plugins in Netbeans 7.1.1.