I've not used either in anger as yet but was intrigued by these for a few reasons. I've been toying with Ruby which Watir uses as to create a domain specific language for testing, I've long felt that automated testing was unjustly the black sheep of the software development family, In a previous life I had some training in ridiculously expensive (yet strangely rather shoddy) proprietary tools that used this approach and finally, I thought such tools could perhaps allow non-coders in the community another meaningful avenue to contribute to Moodle.
Watir is currently Windows only (Windows Internet Explorer only to be exact) which is a massive barrier for me. But it is apparently moving towards supporting Firefox cross-platform and also going to make use of Selenium (which independantly gets good very write-ups in it's own right).
Both tools are primarily for functional/smoke testing and any performance/load testing info they give is probably broad-brush but effectively free if you've already invested for other reasons. I think they could give broad confidence that low-level changes aren't breaking fundamental functionality though I'm unsure of what level of detail you could test before hitting diminishing returns.
