The thing is, they have bloated out the library quite a bit. One of their early selling points was how much smaller the React library was than Angular/Ember. Now the size of react.min.js has ballooned up to 145.4kb, which is only 10kb short of Angular, and with addons it's even bigger.

As someone who focuses heavily on small page size, that's a pretty harsh regression. Especially when compared to all the other VDom frameworks like Riot/Mithril/Cycle which are all much much smaller and achieve about as much (in my opinion).