How many use a laptop?
How many use a tablet?
Thats the potential market, huge. Its not a niche. Two old products melded into one. Tablet heavier, yes, screen bugger, cool.
The consensus among reviewers seems to be that the Surface is a compromise device; neither the best notebook nor the best tablet. It's a replacement device for those who want both devices and are willing to give up something in order to have both functions in one device. That market tends to be business-oriented users, and thus is a niche consumer market.
There are cool things about the Surface. The pen functionality looks nice, and I have to admit that there's an appeal to the look and functionality of it. But in the end I don't find that Windows 8.1 functions particularly well as a tablet, and so the Surface requires too many compromises for me.