This is a great product idea. If you ask any Excel power users, by far the most time-consuming and hard-to-automate task is text and date manipulation.
The beauty of this product is that its adoption strategy is baked into the product itself: I'd share this with all Excel user friends of mine because I want the algorithm to get smarter, and I might even learn a bit of C# myself so that I can contribute and scratch my own itch. This in turn makes the product better (because of the larger training data), lending itself to more word of mouth.
One concern I have is security: I'd love to hear from folks who built this/more familiar with this about how to ensure the security of suggested transformations.