It certainly seems to be rarely used. It's interesting to look at the concurrency stuff in Clojure and its uptake:

- Atoms: very popular. Usually what people reach for, perhaps overly so.

- STM / refs: Not commonly used, but I wonder if this is because we aren't used to really considering them as an option?

- Agents: I think everybody agrees that you shouldn't use these

- core.async: What many people think you should use instead

- Reducers: I've never seen these used in the wild.