I have been using React for more than 2 years. In this time it became one of the most popular Javascript libraries out there. And for good reasons. It is fast, easy to use and it just makes sense.
It is used for building user interfaces. And that is its only and primary purpose. So unlike some other libraries out there, like Angular or Backbone, it is not a full blown framework. It doesn’t give you all the tools you need for a modern web app. You use it to build UI components. For everything else like services, ajax calls or building tools you will have to use extra libraries.