To be honest most of my exposure to RethinkDB is from Aphyr's article on it. We already run basically every alternative he mentions as superior for particular use cases. For example we run Zookeeper / replicated SQL for inter-key consistent actions and Cassandra for an AP store. When we need document semantics we have a pretty robust Elasticsearch setup. It didn't seem like RethinkDB would do all of those things better than those special purpose databases so I didn't really look into it too much.
Replacing Cassandra at Yelp would be a lot of effort, so we'd have to be sure that it's worth it. That being said, RethinkDB definitely looks interesting and I'll make sure it's on my list of datastores to evaluate.
RethinkDB seems to have everything from Cassandra, without the complexity.