We use it for our developer docs (https://developers.goinstant.com/v1). It worked well, a couple hiccups, but did the job.
They've raised a surprising amount of money ($9.2M), but I like the idea of parceling off a specific piece of a website and focusing on that. Search can be pretty lousy on sites, and using Google is just weird. Building it yourself makes no sense. I'd guess there are bigger plans afoot with Swifttype vs. providing a "search widget" to websites--we'll see how it goes.
WebsocketD