I like React because it's closer to JavaScript than any other JS library I've used. The public API is tiny; you get better at React by getting better at JavaScript. The only thing I don't like about React is the rapid churn in architecture best-practices (Flux et al). This is in stark contrast to Ember—which nails conventions and tooling but is written far about vanilla JS. We use React because components are a great…
This is why I'm sticking with Knockout for probably another year or so. It's tiny, safe, easy, stable and it's already been proven to work for large apps.
It also has a beautiful component system and since Knockout simply stays out of your way, you get to focus on Javascript. When the churn dies down in React-land, maybe I'll take another look.