The project is still in early stages, according to Tammie Lister, a primary developer of Underscores Components. There is a fancy new sub-site for choosing which Component to use, and then generating your theme (much like the regular Underscores generator works) to include the specific component you picked.
Right now Components are each in their own branch of the Github repo, but in a later version it'll be a “true generator” where it auto includes the components a user selects and builds the theme from a primary library.
I love this concept. I actually have imagined for some time that a good commercial theme shop idea would be to have a base theme for free, with upsells in a Pro version where the user selects which modules to utilize. I had imagined them as plugins, which presents certain challenges, and the generator concept in Underscores Components is a pretty clever way to go about it.
The repo has been open for a while, but Mark McWilliams (new Post Status member, welcome Mark!) spotted it and posted the link in Slack. The announcement from Automattic is coming on Monday.
Tammie says the project will be evolving quite a bit, and I'm looking forward to seeing what they make of it. You could really go a long way, and I hope one path they consider is more opinionated CSS that better defines layouts to help enable these new components.