The Antec CP-850 is a superlative power supply by almost any standard. Its electrical performance is up at the level of its more expensive brethren, the Signature 650 and 850, and Seasonic's flagship, the M12D-850: Voltage regulation is extremely tight for all the lines at all loads, and the ripple noise is amazingly low.
The noise performance is excellent, with the <400W performance matching or bettering virtually every PSU tested thus far. Above 500W load in our heat box, the noise level goes over 40 dBA@1m, or about the norm for PSUs rated this high. It has the virtue keeping itself extremely cool, however, cooler than any other PSU we've tested at such high loads.
A serious consideration is that in each of the [four] compatible Antec cases ... in the model 1200, a direct path can be maintained to the directly opposite, wide-open front vent. This means that our extreme hot box test conditions never apply to the CP-850; in other words, SPCR's test environment is unrealistically hot for the CP-850. Our atypical spot check with a room ambient thermal test showed the CP-850 would reach only 24 dBA@1m at 700W load in a 27°C working environment. This is ridiculously quiet for such high power output.
The above is an obviously unfair advantage for the CP-850... but what of it? Antec has used an integrated systems approach for its CP-850 and its best cases, and if that approach is an advantage over all other case/PSU combinations, then, all the more power to Antec! It's not uncommon for enthusiasts to frequently replace the motherboard and components that mount onto it — such as CPU, RAM and video card — while the case and PSU are retained. There would be ample reason to take that approach with the CP-850 and one of the compatible Antec cases.
For the quiet-seeking computer gaming enthusiast, the CP-850 (along with any of the [four] compatible cases) is something of a godsend. Fantastically stable power, super low noise at any power load, long expected reliability due to excellent cooling, modular cabling, and all at a price that's no higher than many high end 6~700W models. That you're limited to one of three well-executed high cases from Antec — one mostly for silence (P183), one mostly for gaming (1200) and one that's really an ultimate everyman case (P193) — is not exactly a hardship either.
The CP-850 is non-standard, but compelling, and in its typical application, as powerful and quiet a PSU as anyone will ever need.