At Graphcool (https://www.graph.cool/) we were using Relay since the very beginning. It has enabled us to build frontend products at an incredible speed and while staying confident about the data layer. For instance our entire console is written using Relay at its core. (It's open-source btw: https://github.com/graphcool/console)
PS: We're also the authors of Learn Relay (https://www.learnrelay.org/) which we'll update to Modern Relay soon!