I got the Air just for travel, and I have a much faster 24" iMac... but the Air quickly became my primary machine, which I use 95% of the time! (It's a first-gen machine--fast enough for anything but games, and yet I still do play some UT2004 and Halo on it at low detail.)
It's fast enough, has a big enough screen (for most things) thanks to Exposé, and most importantly: it has all my "stuff" on it (because if it didn't, I couldn't go places on short notice).
So the iMac gets used mainly for Quake Wars, UT, movies/TV, and the occasional big programming project where I like to ponder lots of windows at once. It also syncs my iPhone, just because it has a bigger drive to hold all my media.
The rest of the time--including iPhoto, Mail, surfing, iChat video, and most of my Photoshop and Flash work--I'm on the Air.
(And I sync my bookmarks, contacts, calendars and photos on my iPhone with the Air. The iMac only syncs media and apps.)
And thanks to WiFi, I've also never needed a DVD drive, nor a USB hub
(Until my HD died and then I used a hub just briefly while preparing it for service. I wanted an external drive and my iPhone connected simultaneously.)
When I replace my Air, it will be with another Air. I still have my old 15" PowerBook to compare, and I would NEVER settle for that much bulk again. A computer I can pick up EVEN when one hand is full is worth even more to me than I expected.