This is a decent ultralight laptop -- it's crazy thin, and is constructed out of a block of aluminum so it has exceptional torsional rigidity. Because of this it doesn't feel like plastic junk where you could twist the circuit boards to bits. It has a bright 13" LED screen and full-size, lighted keyboard. The battery is not quickly swappable but it lasts a very long time.
The real compromise on this laptop is mass storage, and to a lesser degree performance. It works smashingly as a web browsing, email processing, spreadsheet crunching type of machine. But if you're doing iPhone development or running VMs or any kind of image processing, you're going to be disappointed. There isn't much disk space and the available hard drives that fit into the ultrathin case are small and run at 4200 RPM. If you opt for the SSD drive, you'll be paying a lot of money to get mass storage with no moving parts, but you won't have room for tons of photos, music or video. There's also no DVD drive built in and only 1 USB port.
In short, if you get a MacBook Air you'll have the lightest, most portable Mac around.
