Overall, version 9 is a nice upgrade: nicer source code repository integration, built in task management, the IDE seems faster, etc.
I use the JetBrains RubyMine product almost every day. Long term, my use of IntelliJ 9 will depend on how good the AppEngine support is. As a test, I generated a new AppEngine project, added some home page material, edited the appengine-web.xml file to specify a registered app name, set the version, then deployed to Google's servers with no problems. I did have a problem importing an AppEngine project from Eclipse but eventally realized that I needed to go to Module Settings -> Artifacts, and drag "Available elements" from the right window pane to the <output root> tree display in the left pane.
I tried writing a simple GWT 2.0 (new version) web app. When I created a new project, no template GWT client and server code was generated - a small disappointment. I deleted that project and used a command line tool to generate a project:
/Users/markw/Documents/WORK/gwt-2.0.0/webAppCreator -out mytest com.mytestI then created a new IntelliJ project "from existing source", and had no problems using the generated template project, running the local development server, and using the web app in Firefox (after installing the GWT FireFox plugin).
I am really busy with client work right now but I hope to get some time next year to do an AppEngine based project of my own, and I am almost sure that I will use IntelliJ 9 for that because general Java development is a bit better, I think, than using Eclipse, and it looks like the AppEngine support is OK.