Firstly, as expected, the experience of having it in your hand is vastly different to reading the spec sheet. You cannot imagine the MacBook Air by holding a regular MacBook and trying to envisage it being thinner and lighter. The difference in weight is not one of degree but of kind. The entire machine can be trivially lifted, open, in one hand. I can't even one-hand a MacBook with confidence, but wouldn't give a second thought to the Air. Closed, the Air is almost ... not there. It's almost like it melts into the tabletop when you close it.
One thing I'm not sure I like is the narrowed footprint: the way in which the keyboard overhangs the base of the machine. Being so light, the thing felt kind of tippy when open.
Otherwise: sweet machine. When they figure out how to get a 160GB drive in there, I'm going to have a much harder time walking out of the store without one.