We use PhantomJS as a headless browser, sometimes taking screenshots. I know, crazy. Anyway: http://www.mystartup.com
It has a few lingering bugs (for instance, some issues with file uploads), but overall, it's a nice improvement over 1.9.x. The JavaScript engine is much newer and the rendering is improved.
We, AppNeta, use PhantomJS to perform Synthetic web-app monitoring
http://www.appneta.com/products/appview/
While PhantomJS has some limitation here and there, it is the _only_ headless browser our there AFAIK.
Disclaimer: I work for AppNeta (specifically for the AppView product).
By hostile I mean that the legacy system had no API, was implemented using obfuscated code, and resisted being driven by URLs.
We needed to integrate with it to create documents, calendar appointments, etc. in the legacy system, based on stuff that was happening in the newer system we were building.
We ended up using PhantomJS to implement a module that drove the legacy system via its HTML user interface -- pushing buttons, selecting meeting participants from the HTML select tags, submitting forms, etc.
Although that sounds (and is!) hacky as hell, it worked flawlessly 100% of the time. PhantomJS was a gift from the gods.
But that's one use case where I would love to use phantomJS or any other headless engine to test the frontend. I'm aware that for headless testing to work, there should be a headless browser of every major vendor. Maybe it's an utopia.
Being able to use websockets in my automated testing has been something of a holy grail, so jquerygo and phantomjs have been hugely helpful.
Edit: it appears to be WebKit 538.1, from May 2014. But this is based on a readme which is pretty old, so it could be outdated info
A big thanks to PhantomJS devs!