I have two, had three. I've upgraded the CPU, the RAM, the video card, the hard drive, about to upgrade the VRM and probably tap out at 1.6ghz here soon enough. I've changed the case on one to the all-aluminum Powerlogix "supercube" case, but kept the other one in the stock case. And I've added Airport to them all. As well as added superdrives to them.
Not upgradeable?
Yeah, I can't change it to USB2 and Firewire800.
When it came out, it failed big time because it was just too darn expensive. 1.8k to 2.2k for something that was quickly outpaced by it's Powermac brethren.