From what I've observed, most MIDs are too big, too expensive, too fiddley, and have short battery life. MIDs won't take off until you can easily carry one in your pocket, buy one for less than the price of a cheap laptop, use it without knowing how to use a computer, and count on the battery to last all day. It'll help if the MIDs have mobile broadband and aren't locked to a single carrier.

By this standard, even our beloved NITs fail, alas.