Opinions on A/C Noise Meter EMI Testers

A local dealer has a plug in device that gives a numeric readout of undefined scale or meaning, but is helpful in determining the rough scale of noise problems and how much each change has improved the conditions.  It has a speaker that also outputs a demodulated version of the noise (turning ultrasonic noise into something one can hear such that RFI can actually be heard as a radio broadcast).  Even after putting in a power conditioner, a customer had noise issues detectable with the meter which was helpful in finding the primary source of the noise--stuff was unplugged around the house until the noise source was found (a string of LED lights in a bedroom on a completely different circuit from the stereo).

I have found mine useful, it is a low cost RFI and EMI detector I bought on Amazon. I used it when building a custom streamer to test the shielding. I also use it to test components and power cables. They all emit something, but it is useful in finding items that are emitting more than their fair share. 

I think they measure something, but I've never seen of one fixing a problem a user could hear.

I've heard of users who could hear LED power supply problems, didn't need a meter though, it was pretty audible.