Wouldn't that be only for power users? I am very good at Excel myself but it's not part of my job to work with it so I'd say most of the time I use Excel as a quick storage for data and not as a calculation sheet. Grid sounds great for almost every case where you would fire up Excel but not actually do any complex calculations. It made me think of this article by Joel Spolsky about Trello (and spreadsheets): http://www.joelonsoftware.com/items/2012/01/06.html
Almost everyone who uses a spreadsheet wants to do basic arithmetic in it, though.
Look at the demo video on the site. There's a list of expenses and dollar costs. But you can't automatically sum them? Anyone familiar with spreadsheets would consider that a pretty fundamental feature.
