Although my title looks a bit dodgy, it covers what I think of the product.
Having the Yamaha electroacoustic, for recording purposes (if using the jack) this is a bit of a must, especially if recording vocals simultaneously, as you dramatically reduce the volume of the guitar.
For playing the guitar where you need to be quiet, i.e. after the kids have gone to bed, it is also really useful, as mentioned above it dramatically reduces the volume of the guitar without and real effect on the tone.
Once fitted, as the product has a double lip, there seems little chance of it falling off as it is quite snug and secure.
At the end of the day it is a oval shaped piece of rubber, but it is for a specific guitar brand and it is well made and does the job well, so I can probably justify the price.