It's one thing to see a variable fonts demo (oooooo one font can change things like weight, width, and slant?) but it feels a lot more real when fonts you see and work with all the time go variable. Adobe made six of them available: Source Sans, Source Serif, Source Code, Myriad, Acumin, and Minion. You can't serve them on the web directly through TypeKit yet, but you can download them from GitHub to start playing.
Print designers have just as much reason to be excited, or perhaps more, as so long as you have software that supports variable fonts, you can use them right now: