Don't use this for anything more then a playful lib. It's fun, and it's helpful for defining what browsers might want to do in the future, but its a toy, like a fisher price hammer.
Actually, there's no reason you can't use this on a real site. Sure, you don't want to use it on a long block of text, but it's great for headers and short copy. It doesn't break anything and the "fallback" if it doesn't work is just unkerned copy.
Yea, but there is flash of un-styled content. There is no way around it because the style has to be applied by the javascript. That is not production ready in my opinion. The way standards bodies work now though I think this a great way to demo what browsers should be able to do.
I should note this in the documentation, but if you add "visibility: hidden" to your content, it will automatically be set to "visibility: inherit" after it's ready.