ReactJS is good
For a lot of front-end developers, jQuery lowered the entry bar. Other libraries like KnockoutJS (MVC framework), MarionetteJS (view framework), BackboneJS (model framework), Handlebars (templating) and AngularJS (MVC framework) made building large-scale applications really practical.
ReactJS is relatively new to me, having worked with some of the other frameworks for 5 to 8 years. As always, I decided to build a new application to really determine what React can do for me. So I spent a few evenings building Free Q and A (http://freeQandA.org).
React is elegant and powerful. One thing React really excels at is building components. React did not invent this concept. It’s possible to do the same thing with Knockout and Angular but the learning curve is quite mild and the simplicity of the framework is commendable.