I just got one of these, and I have to say: Wow!
It silences single coil hum. Totally. I can now dial in different volumes on the bridge and neck pickups, and there is no hum. My single coil single pickup Peavey Patriot is dead quiet.
It's got two settings: normal and strong. The normal setting is what I use. The manual says it eliminates the odd harmonics from the hum. The strong setting eliminates even and odd harmonics. I can hear some color on the strong setting, it rolls off the highs a bit. But the normal setting is as transparent as a true bypass, to my ears.
It doesn't eliminate other electrical noise, but it does an amazing job on single coil hum. The drawback is that it's fairly pricey, and it only works with the included wall wart. It can't be powered from a battery.
But I'm loving my clean single coil hum-less sound now.