When I investigated React in detail, I found that there is a lot to like about it, but it's certainly no "holy grail." Specific downsides: you have to go back to manually writing event handlers (instead of getting automatic two-way data binding) and the virtual DOM is a leaky abstraction.

Here's my article where I analyze this in much more detail: http://blog.reverberate.org/2014/02/react-demystified.html

Also React does not "solve all the problems in the world" -- it says nothing about routing, validation, testability, and many of the other problems that Angular has solutions for.