Mribob,
I went to a dealer's last weekend to demo a pair of Elac super tweeters. I noticed the place was filled with all these boxes from Sweden and I asked my dealer what these were. I had never heard of Entreq or 'grounding boxes' but he offered to give me a demo so I said sure.
As you say, they remove grunge, and provide quieter backgrounds and deeper sound stages. What sold me personally on this product, however, was the fact that so much more emotion and inner detail comes through. Everything becomes more relaxed and more 'musical.' You get a better sense of the flow of the music and it is much easier to sense the intention and emotion of the performers. I feel it is worthwhile but it certainly isn't cheap. My girlfriend was skeptical when I brought it home but she couldn't believe the difference it made to the ease and flow of music through my system.