Author of <a href="http://www.amazon.com/exec/obidos/ASIN/1932394230/ref=jranch-20" target="_blank" rel="nofollow">JUnit Recipes: Practical Methods for Programmer Testing</a>
I am a believer in TDD, but as a practitioner I tend to slip sometimes. When it comes to GWT I tend to slip more than usual. ...But I am working on it.
As for test-driving, I assume you mean the JUnit support in GWT? I will say that it isn't as smooth a ride as I would like. First you need to use only those classes that you can use in a normal GWT app (read limited JRE and Java 1.4 syntax), because the test case, like the GWT app code, will be compiled to JS for testing. Second there is no support for suites.
On the flip side, the one thing I really like about it is the ability to test the RPC calls between the client and server. When running the tests GWT starts up a light-weight Tomcat instance for your server-side code, then in the test you can call your client code to trigger AJAX-style RPC calls against the server. I don't know of any JS tool that will let you do that.
I think that the JUnit support is great, but it could be better.