We've been trying it out it as a replacement for Firebase and DynamoDB. It's great! The (JS) API is well thought out and it's easy to do server stuff with it. I miss some of the shinies from Datomic but administration is much easier.
We have a cluster of master DB servers running on compose.io. Our AWS EBS application servers spin up both a node app and their own copy of Rethink running in proxy mode. We can spin up any number of application servers, while keeping the load on the masters super low, for great HA (our setup is read-heavy with low writes).
Going forward, any new client-server project I work on will probably be Rethink-based.