It's fun
Aside from the lower resolution offered by the PowerMacs (at least potentially), the iMac is now an awesome machine. My 400MHz G4 will last me some time, as will my iMac DV at 400MHz, and now I've worked with the new iMac. It will be considered fast for a good two years, and that's great. Given that it features a G4 processor, the iMac can run any professional software, and well. Heck, even Final Cut Pro 3 will do certain things in real-time. Although, with only 1024x768, Final Cut Pro 3 wouldn't be very fun.
The new iMac is a bargain, and can handle any professional applications out there. Until the PowerMac line picks up some steam, I wouldn't even recommend the PowerMacs, unless a cinema display or a trifle more screen reality is necessary, I say buy an iMac in a jiffy.