They way I see it, tablets are most useful for commuters watching media and e-reading. If I needed the portability and battery life in a plane or train, it'd be golden. I do no commuting, so I have no use for one. I have a nice laptop next to my lazy-boy. If I need media, it's right there. And I can play the occasional heavier duty game, as needed.
My old Droid1 does bathroom/bedroom duty just fine, and it'll be that much better once I get my Galaxy Nexus.
My brother had shown me his iPad, and I was minorly impressed. But everything he was showing me was stuff laptops had done for ages... Nothing mind blowing...
Again, they'd be golden for commuters and e-readers, but I don't get the 40 million statistic. My bosses have been checking them out, so I guess there is the business/toy aspect, but to each his own, I guess.