I took some time last week to play around with ReactJS, and I created an OWA example at https://github.com/djazayeri/openmrs-owa-reactexample
If this is something we want to support, we need to make some changes to the yeoman generator (which currently lets you start with jQuery or AngularJS), e.g. to add the react preset to webpack.
All in all I found it pretty easy. ReactJS only handles the view/presentation layer, and it makes it easy to write components (actually, you can only write components). Based on this very limited experience, I think I prefer react's model of "writing JS functions that return HTML" as compared to angular's usage of ng-show/hg-nide/ng-repeat.
Next I will refactor it to use a Redux store, so here is a permalink to the current simpler code: https://github.com/djazayeri/openmrs-owa-reactexample/tree/caadf15be33ff26aabe47eb3600f65fd1043530c
FYI, on the Bahmni distribution our plan is to migrate incrementally in the direction of react, and use redux to get a better handle on having shared application state that can be modified by different pluggable apps and components (some of which are legacy ones written in Angular 1). And we've started our first efforts in this direction. We'll share more once we have something demonstrable to show!