The JHS Little Black Amp is a fantastic device for just about anyone to have in their arsenal, especially considering how cheap you can pick one up, it's something I use often and for a couple of different potential purposes.
THE PROS:
Simple and effective. The device attenuates the signal right before it hits the power amp reducing the volume of loud tube amps for those occasions when you need to be quiet and still sounds fantastic.
The device can also act as a kind of pre/master volume if you have multiple master volumes for your channels or other preamps which you want one global control for, which is super useful!!!
THE CONS:
None really, maybe some level markers would have been nice. We just used colored pencils to mark our favorite positions, no biggie and way cooler anyway ;)
A GREAT option for those looking for cheap, reliable good sounding form of amp attenuation, would absolutely recommend.