The market isn't a good test either, for at least two reasons:
1. If the market was the test then any product that was primarily intended for individual consumers would be toy. Clearly, that's not how we judge products.
2. Businesses, especially large businesses, are slow to adopt new technology because it is expensive and risky to switch frome one platform to another. After being on the market for only 3 years, the iPad accounts for 11% of all enterprise sales. That's a small number but given the newness of the product it is significant rate of penetration. Looking at a snapshot while ignoring the trend doesn't provide an accurate story.
The iPad (and tablets in general) is a computer, just in a new form factor. Like traditional computers, it is a device that can be used for productivity, for learning, and for entertainment. It's a tool that can be used in various ways to achieve multiple ends.