The Zalman ZM-850HP is not just another 850W power supply. It is engineered from the ground up to be silent and efficient. Two heatpipes on the inside transport heat away from the hot components to the rear of the PSU where it is exhaust by a big slow running 140 mm fan. Since the PSU's efficiency is above 80%, Zalman can afford to run the fan much slower because there is not so much heat to get rid of. This results in an extremely quiet power supply - actually the quietest we ever tested so far. For a PSU with 850W this is extremely impressive. Now you can not only build a powerful system but also one that is quiet.
Zalman has also thought about the modularity concept some more. The important cables like ATX power, CPU power, two PCI-E video card connectors and a few peripheral cables are fixed to the PSU which means there won't be any losses in power quality which usually happen when a modular connector is present. Should you need to connect more devices you can use one of the numerous modular cables included.
There are only a few minor things that I can complain about like the lack of a real power switch on the back of the unit. This is a part that costs several cents, I fail to imagine why this was left out. Since Zalman has split up the power into six rails with 18 amps each (216W), there could be some situations where one rail gets overloaded in the most extreme overclocking situations - for example each video card drawing far over 500W of power (216W + 216W + PCI-E bus power). Not likely to happen.
Even though the price of $250 seems to be justified it is still a big chunk of money. However, having a stable power supply in your case is very important. If you can spend the money on a system that needs an 850W power supply, you should be able to afford a good quality PSU too.