I haven't looked at the actual spec, but from what I've read on the subject it seems really exciting! I think the most exciting thing about them is that they're live variables, versus precompiled variables like Sass. That means you can dynamically change values during runtime to accomplish some really cool stuff.

Obviously, these custom properties can help maintain stylistic consistency across a site, but it seems like they can do more than that. Jake Archibald recently talked about how to use these properties to more thoughtfully load in CSS partials. That's just one example, but I have a feeling we're just scratching the surface of what can be accomplished with them.