I almost always use the iPad while I am laying down on the bed and resting the iPad on the upper part of the stomach. I find it gets tiring holding it in the hands for any length of time.
When I am at the desk, I almost always use the desktop and sometimes the laptop.
Don't get me wrong, I am not lazy and would like to think I am quite fit as I exercise regularly, but I suppose if people can play games with game controllers for hours on end, there should be no problems with the iPad. But for me, I just don't find it comfortable using the iPad in any position other than lying down.
Of course, what I said above will go for any tablet device and not just the iPad.