The only drawback right now is Photoshop CS is not available yet in the Universal binary format so PS runs a bit slow. Everything else just flies. Been editing/playing back HD on a 2 ghz macbook Pro connected to an 23" cinema display (functional equivalent of the iMac you're talking about) and it's just been flawless. Plus you can boot it up in windows or run windows in a window and have a fully functional windows machine as fast as the comparable PC boxes.
When I bought my first new style 23" cinema display it cost $1999 so in my mind that 24" iMac is a heck of a bargain. If I didn't need the portability I'd be all over that unit. If you get it put as much RAM as you can afford in it.