I've had the Technowarehouse upgrade for two months now. After an initial problem with kernel panics and freezes, it now works perfectly. The contact between the chip and the heat sink, which is mounted to the chassis, is critical. On my computer, the contact was just slightly off square. The fix for my problems was a postage stamp of rubberized conductive material on the chip to insure good contact with the heat sink. Jack Putnam, at Technowarehouse, tells me that all upgrades now come with this.
I have to use several PC programs in my work, so Virtual PC is essential. Before the upgrade, I was ready to buy a new Powermac because VPC was so slow on the iMac that I could hardly stand it. The upgrade runs VPC at near normal PC speeds. The difference is unbelievable. The difference in VPC alone is worth the price of the upgrade.
After the heat sink fix I did an archive and install of 10.2.6. Since then, I have run trouble free. No crashes or freezes. Everything runs much faster; iChatAV works perfectly (no iSight yet). Substantial speed increases across the board. No problems with sleep. I don't do video editing, so I can't comment there. I have had no problems with compatibility; everything I have tried to run works fine. I am not a gamer, so the lack of an upgraded video board is not a problem.
I am totally satisfied with the upgrade. I can't say enough good things about the service from Technowarehouse. I would definitely buy the upgrade again. Money well spent!