For those who want to know, I bought this fan for a 'home server' set up I am currently working on. I already had the stock hp 120mm fan which kicks out more air then a leaf blower, and is noisier then one!
I replaced it with this baby (and also a Akasa exhaust) to try and minimize volume without too much a drop off of air volume. Now one thing that this fan DEFINITELY is is quite. Quieter then the 80mm front fan. Quieter then the CPU fans and quieter then the PSU fan. Which is brilliant. Over the rest of the fans in this thing you cannot hear it whir away at all. Perfect. Noise level of the server has more than halved.
HOWEVER, the volume of air this thing can kick out the rear of my server is less than half that of the original HP fan. The original running at full speed could blow paper across the room. This one is a pleasant breeze at best. But it is enough! With the exhaust (also incredibly quiet) it keeps the server cool enough without anymore then a little hum.
I am pleased with the product and would recommend, long as you don't have a beefy graphics card pumping out tonnes of heat (or you have some other form of cooling in there as well).