I'm writing a high-performance webapp that requires some specific data-structures and algorithms to achieve the speed needed. I tried prototyping in RoR first, but even simple test requests ended up taking minutes to answer. Then I conveniently attended a guest lecture by Rob Pike on Go, and had one of those all to frequent "oh cool, gotta try that" moments. I must say I don't regret it one second though. Go is a surprisingly nice language for writing all kinds of things in. The features of the language combined with the rather extensive set of standard libraries, (IMHO) makes it ideal for high-perfomance webapps.

I tend to do these thing just as much to learn new things though. So I'm not saying it couldn't have been done in another language instead.