How does it differ from PhantomJS/CasperJS (headless webkit)? I have been using these for a while and they can do pretty much everything that you can do in a browser.
I have a 1/2 written post about this exactly. I took it out because it would have made this post too long. But TLDR: I found a few edge cases where using a real browser was easier to use than a headless toolkit. Definitely will be my next post.
The point of testing in a real browser is to test in a real browser because your users use a real browser. Testing in a headless browser is faster, but proves nothing.