My fascination level doubled when I realized there was no backend to this software. The state of what has been typed on the screen is stored in the URL, compressed and base64 encoded. When you send a link of your document to someone else all of the information is stored in the URL, and thus they see what you see.
This state-in-the-URL is handy for my financial tab group. When I sit down to pay the bills (RE: pay the credit cards), Numbr is the first tab I see. I don’t need to dig through a service’s interface to find my document or spreadsheet. It has all the miscellaneous calculations I felt I needed the last time I worked with my finances. I can tweak and edit the document from there, building it upon itself.
It’s a novel approach to building software. And it’s pretty.