This has been the first upgrade on my little project so far. Currently still using a Pair of 6" 18 Sounds (Changing to Alpine SPX-17Pro) on the door until I acquire all the other components to begin, so my reviews on it is based on the 18 Sound.
Considerable improvment in the Clarity of the Vocals of different Genre of Music.
Was able to set crossover HPF lower from where it was to a little below 80 HZ and still sounded cleaner and clearer. Prior to putting the Mat, 120 Hz was probably the lowest I could have gone where the mid bass started to sound distorted which for some reasonaffected Vocals...
I have bass upfront

Music blends in much with the sub in the back and speakers up front sounds fuller. "Imaculate Collection" have never sounded so good in my car before.
Small drop in External Noise, nothing much to talk about, but when added to the increase clarity in the sound it is quite noticable.
I would recommend this to be the first thing anyone does to upgrade any music system. I am sure even stock speakers will sound a lot better with sound dampening... It is not overrated...