Having owned the ipad 1, 2, 3, 4, mini, and now having bought and returned the Air today, my takeaway is this; the Air is one sexy piece of kit (assuming you don't have a 1/2 pink screen like I did). If you never had a mini, you will think the Air is the greatest tablet to date, and the end all be all.
But if you're like me, once you go mini, you just don't go back.
The mini is infinitely more ergonomic. It was made for hands and fingers. The Air is made for hands and fingers, but also requires wrists and arms. From a resting position, in portrait orientation, reaching upwards to interact with various elements at the top of the screen quickly becomes a repetitive chore, compared to the mini.
Handling the Air seems breezy at first, but it soon devolves into an exercise in load balancing and shifting, as it becomes tiresome to hold in one position for too long. The way you grab it when you pick it up for the very first time and marvel at its slimness and lightness, is ultimately unsustainable. It is deceptively weighty the longer you interact with it.
Weight and size aside, from a usage standpoint, the mini is truly a personal device. In the few hours I had the Air today, I felt like everyone was peering over my shoulder at it, checking out what was on my screen. The mini is a totally private affair.
Finally, no single aspect of the mini brought such a sigh of relief upon returning to it after using the Air than did typing. Typing on the mini in portrait mode is like typing on the iPhone, only easier, to see and do. Typing on the Air, while much better than previous full size models, due to the thinner bezel, still feels more like checking in at an airport ticket kiosk than typing. It's slow and deliberate. It's hunty and pecky. And typing a long email could mean your hands travel up to a mile across the screen and back, especially in landscape mode, and especially if you're like me and not a touch typer.