OK so when I brought this I actually wasn't paying attention and was expecting a 5.25" fanbus, so I was a little shocked when it arrived and was designed to fit in a 3.5" floppy bay. Luckily my new case had such a slot, and since I had no floppy drive to put into it this fanbus was fitted instead.
And I have to say it is a pretty nice piece of kit. It has no lights on it, so it doesn't draw attention to itself or distract you, and the knobs on it turn smoothly. I am currently running two Nexus 120mm fans off it with absolutely no problem at all.
The knobs on the front don't allow you to turn the fans fully off; I'm not sure what kind of voltage swing you get with the pots but I would guess somewhere between 7V and 12V since even with them turned all the way down, my fans keep spinning (and 7V is about the minimum that will drive a Nexus 120mm).
The fitting was easy, it came with enough screws and properly labeled cables and converters and the metal has a nice finish too. So from me there are no complaints what so ever. If you're looking for such a device, this one has no drawbacks that I can think of!