The ZM600-HP is clearly targeted at the high performance gaming market. The high power output, good voltage regulation, and excellent power factor correction make it a solid candidate for a new dual core, dual video card setup. The price tag also matches that of systems that might have such a high power requirement. The modular sleeved cables are also another nice touch for those that demand tidyness or want to show of their rig to their gamer friends.
Unfortunately, a gaming rig is typically not the realm of quiet computing. The unit performed quietly only at < 150W load, and would most likely be amongst the loudest components in a typical "quiet" system.
It's strange, since Zalman's product pages are littered with talk about "ultra-quiet" systems — maybe they're talking about ultra-quiet relative to the typical high-end gaming rig. Their ZM460B-APS power supply boasts a feature Zalman calls CNPS (Computer Noise Prevention System) that is supposed to keep the fan running slowly until the internal temperatures reach 30°C, but the ZM600-HP has no indication of such a feature on their website. Our results suggest that the absence of such a claim is no accident. One possible source of the problem lies in the tight spacing of the heatsink fins on the heatpipe. With such a small gap between the fins, the fan needs to spin faster in order to provide sufficient pressure to sufficiently cool the device. For high airflow systems, this provides more efficient cooling, but it significantly reduces performance in low airflow situations. Although the heatpipe heatsink is interesting, it has not helped Zalman to produce a PSU that uses less airflow for cooling. Or one that's quieter than the competition.
In the end, if the ZM600-HP is used in a system where the CPU fan, hard drives and/or graphics card are loud, then yes, you could call this PSU "ultra-quiet". If silence is your goal, there are several other PSUs even in this high power category that are better choices