I use a 17" MacBook Pro from work (and lug it to client sites and home as needed). When I'm "off work" tough, I'd rather there be a clean break between "me" and "work" (often leaving the laptop at home, sometimes merely because of its size). I've grown accustomed to having a laptop around and there are times it would be beneficial, but the weight, size, and replacement cost usually preclude bringing the 17" with me to a lot of places.
I've got a desktop in my home office for when I'm there, but if I'm by the TV in the family room, on the road or on a vacation, I expect that tossing my Mini 9 in my travel bag should be about as difficult as bringing an overlarge hardcover book, and much more useful than my old Palm/Clie was. It will also be a small handheld console taking the place of some of the smartphone lust I've managed to avoid (I'm happy with my older "brick" phone, it works and its good, it doesn't need to do everything until they get battery life MUCH better

).
That I can use a netbook as a personal organizer for my contacts, browse the net at WiFi hotspots, offload camera memory cards on vacation, or use it as console for a bit of programing while sitting on the train, all while only carrying around a 2lb oversized hardcover is pretty amazing.
The mini sold me on the idea (over any other) because it was small, light, and could theoretically be turned into a hackintosh (using a standard install disk and a VERY clean upgrade capability), giving me a computer only twice the weight and slightly bigger than the Mac Air, but still supporting (and providing) almost all the same basic features one would "expect" in a traditional OS X computer (bluetooth, 1.3MP Camera, 802.11g). It also provides some features the Air doesn't have, such as a built in multi-card reader, WWAN optional, lots more connectivity ports, oh, and even a fully loaded mini9 is 800$ cheaper than an Air.