I use it on my Tornado 92mm fan with a toggle switch. It's like night and day. But, there's also a 8 degree difference in temperatures with it off (8 degrees higher) according to my case sensor which is on my harddrive and not near the fan.
Yeah, like G said, some fans may not be able to spin up at 5V. None of the ones I use can. If you want to undervolt them, it seems that 7V is pretty good from a performance/noise perspective. Decreases noise dramatically, and still will move a lot more air than it would at 5V.
It is in the FAQ under case modding. Basically intead of using a 12V line and ground (yellow and black, respectively), you use 12V and 5V (yellow and red, respectively) to create a potential difference of 12V-5V, or 7V.