Thanks for giving it a try @nathank! It’s still considered experimental and we will make changes to the API, so I wouldn’t build a homepage-builder-type theme component on top of it for now.
The demo blocks are just basic examples. We also have some API changes coming up that will improve how we can load data. I’ll push changes to the all the blocks on the demo theme once this is available.
The category banners block should show on all categories. I think the category picker you reference is for the featured categories block on the homepage.