We were looking for an API that serves up-to-date currency rates in order to implement a currency-conversion feature for Kittysplit (a side business we are part of). We compared a few different APIs and integrated one of them. It worked well enough, but there were a couple things we didn’t like. First it was REST only, as was the entire competition. A secure connection was only provided for paid plans. Response times were often slow and some other annoyances.
Nevertheless we started integrating and using it, but kept in mind, that a currency rates / conversion API that meets all our demands would be a nice future side project for us.