I used it for a project, along with a simple Flux implementation, and it did the job well enough. The declarative, state-based aspect of it is great. This might be a small thing, but I also missed their unified onChange event when I moved to another tool.

I didn't really like JSX, though, and using straight-up JS to declare components is gross. Using React in general also seemed rather verbose, and I found myself digging through the API docs and trying to reason about the component lifecycle. It just wasn't as simple as I would like.

I ended up settling on RiotJS, which shares a philosophy with React but is closer to bare metal, so to speak. It feels more like a library and less like a framework. (They also just added mixins and HTML transclusion, two things that I missed from React.)