I couldn't possibly describe how much I love the iPad mini. It's just a companion device, it isn't always the right device, but it's a real joy to use when it is and it's so damn portable that I always have it on me for when the right times arise. With the full sized ipad, i just found myself not wanting to bring it with me, not as much because of weight, more because of general bulk. Picking one up after using even the older mini for a year...the full sized ipad feels absurd and unnecessarily large.
This is one area that the Google folks who fast followed after Apple got right, although I don't think Apple could have released an iPad mini when they released the first ipad. There was a very real problem back then of "isn't that just a super sized iphone?" that needed to be overcome to establish the market. We were treated to so many mocking photos of people holding ipads up to their ear, if the mini had been the first gen I think it may have been ridiculed into failure. Weird digression from the discussion at hand, but it seemed almost relevant.
In any case, I think the mini's size is perfect, with the probable exception of being unsure of whether there are any keyboard cases worth using, if you have that requirement. It's awfully small for keyboard viability, although I think clamcase is making one and the later zagg's might just make it plausible. I suppose if i were going to try to use ipad as a primary computing device or even a majority computing device, I'd probably need the Air to give myself a bit more flexibility.