I'm in Costco at least once a week and walk by these things wondering if I should pick one up. The deal is good, and it includes 3 years of AppleCare, which really makes it attractive.
Right now I'm running a G4 iMac that does everything I need pretty well, so I can't imagine that moving up to a G5 would be a disappointment relative to the Intel iMac. Some of us just don't need the latest and greatest device.
That being said, it also depends on your real need. If you don't have to have it now, I'd suggest waiting until the Intel version settles down and you can get the next generation of devices that are better supported by Universal Binaries. That's what I'm planning to do, given that most of my tasks, especially on my home machine, are limited to fairly basic functionality. Even my Photoshop endeavors are pretty harmless.