It's great!

In terms of concepts it's very similar to MobX but the API is minimal because it's focused on react only.

We're using react-easy-state in the Frontity framework https://github.com/frontity/frontity/ and very happy with it.

(shameless plug warning) I've written about principles behind react-easy-state and MobX here: https://czaplinski.io/blog/make-your-own-mobx/

Seems mainly like an alternative to using class-based stores, which I guess gets under some people's skin.