I'm gonna go ahead and avoid repeating what's already mentioned in vjeux's presentation (https://speakerdeck.com/vjeux/react-css-in-js). Instead I'll focus on defending against the common "shortfalls" people tend to immediately point out with this approach.
Designers will be uncomfortable with learning this: React's JSX has shown that (surprise surprise) designers can bear modifying markup with a little bit of code mingled in it. In this case, JS objects in their simplest form map very well to css declarations. I don't believe designers will be scared away by this at all.
"Atwood's law", "we already have CSS", "stop reinventing CSS": no one's reinventing the idea of CSS; we're reinventing the implementation of it. Declaratively specify your layout is really nice, but you do want the backing of a real language to do so. Merging two styles is just a `merge()`, Overriding a style is just `bla.color = newColor` (assuming you don't mind the mutation; otherwise clone it or use persistent collections), and (!) reusing, importing, exporting styles is already solved with commonJS since you're working with normal js objects.
We already have preprocessors: except preprocessors are there to solve the exact problem "CSS in JS" is solving. The difference is that the latter, again, uses a real, familiar language instead of inventing an ad-hoc new syntax like we've done countless times with HTML templates. And since preprocessors need to generate the final CSS beforehand, this limits lots of potentials such as looping more than, like, ten times.
On this last point, I hope that none of the critics here used React's virtual DOM before; If you did, and are still complaining about writing CSS in JS, then you should go back and re-learn why the virtual DOM came to be. CSS in JS is the "virtual styling" equivalent for CSS. The potentials are huge (runtime feature detection for example). And this new layer allows us to map the neat, declarative style to something else than CSS: say, webGL. This is platform-independent; if anything, it's a nice conceptual model.
(Disclaimer: I made https://github.com/chenglou/RCSS)