As someone who is fanatical about both Visual Studio and Raspberry Pi I think this is awesome. It likely won't be a big seller though.

People use the Rpi for hacking and tweaking, and VS is better suited for enterprise development work. You're basically using a space shuttle to drive to the grocery store. Many of the things that make VS great aren't really going to apply to the Rpi anyway.

Most of the "apps" that will built for the pi are going to be homebrewed stuff for education and I think you can build most of those with conventional Linux tools. But if you already have Visual Studio and your comfortable with it, investing $100 in this program would probably be worth it if you want to build a serious pi app.