I didn't walked this path (from ng to react), but to get a solid understanding, the book by Juho Vepsäläinen survivejs.com helped me allot.
React itself is super easy, it has only a hand full of functions. It's just Javascript :-) The tricky parts are around React, namely the build systems, JSX and unlearning all the two-way-data-binding habits.
Build systems are confusing because they are complex in many ways. In SurviveJS Webpack webpack.github.io is described and used.
JSX is just HTML. The tricky part here is, to learn when and where to put repeating parts. How to code a list or table and how to compose components from other components. But with 'stateless components' it has gotten allot easier.
Unlearning two-way-data-binding habits are critical and depressing sometimes. But you are forced to unlearn it anyways (regardless of sticking to ng/Ember or whatever) because observables aka Object.observe() just got removed from ES7 :-)
In React a pattern called Flux is used to overcome limitations introduced with the unidirectional data flow. But Dan Abramov has created Redux and explained it in this fantastic video series egghead.io/series/getting-started-with-redux He also teaches you some more function Javascript :-) Flux will be replaced next year by Relay. And Relay already looks very much like Redux (from Dan Abromov).