I just got the same thing in the Intel version iMac.
Overall, its a bit snappier on the installed apps, but nothing to rush right out and get one over. I'd sure not get one yet if your primary apps arent native, though office seems to run at about the same speed as the G5. I set it up identically to my G5, with the same music, settings, photos etc. and tested them side-by-side, timing with my trusty nano stopwatch.
> The annoying startup sound comes on in various volumes, sometimes reasonable, sometimes full blast
> Screen colors slightly more accurate than my G5, yet screen seems less "crisp" than G5 (could just be normal manufacturing tolerances)
> The "Migration assistant" was worthless - had to manually copy music, photos, email/mail settings etc...
> iPhoto6 crashed trying to import photos, so I had to manually drag them into the library (that worked ok)
> Computer only comes back out of sleep about 50% of the time. When it does, about 50% of the time my airport network is gone - not just off, but missing - there is no link to it anywhere. Looks like the Microsoft solution of rebooting until it works is now a mac 'feature' too
> System prefs app corrupt - often crashes or doesn't save. Can't remove the volume icon in the menu
> Direct airport connection doesn't work at all. Works on G5, but no connection on intel box - had to use firewire cable or bluetooth for file transfer. (connection to my PC network to get to internet fine though)
> Had several crashes in iPhoto and Safari, just doing normal stuff.
> Hardware test won't run - hangs at selecting language
Short of a total wipe and try again, I'd say I'm getting early adopter syndrome: excited by the new tech, let down by lots of problems that I didn't get on my G5. Hopefully the next few OS updates will improve things and show more performance improvement.