I'm with Paul on Selenium being very easy to setup and start working with. I use Selenium IDE in Firefox with some customization at that level, then you can export these to other platforms such as Java JUnit tests. It was quite easy to download and launch the selenium-server.jar, add the selenium-java-client-driver to my existing Eclipse Maven POM driver project; then launch the same exported JUnit test in Eclipse. I mainly wanted to use Java just for looping which the basic Selenium IDE didn't support.
I have configured JSF Unit for my project too which does require more time to configure... more importantly though with in-container tests like JSFUnit changes to the test require rebuilding the WAR, redeploying in the container and then executing from Eclipse or via a browser. So for quickly trying a small change this is time consuming. Of course with JSFUnit you have access to all the internals of the JSFSession etc so it depends what granularity of testing you need I guess.
I'd be interested if anybody knows a faster way to turnaround changes to a JSFUnit test and execute it. Definitely Selenium tests feel more like JUnit tests in that regard.