We did review the Furutech a while back in issue 16.
I won't entertain any arguments about all the science behind the Furutech, because I'm really not qualified. I just know that when I walked into the Furutech room at CES two years ago with a good friend that has a Masters in Electrical Engineering and a PhD in Physics, we were both extremely skeptical and were ready to harass the presenter.
But, we heard a very marked difference between discs treated on the de mag and untreated ones. However, I must admit I've never been able to hear a diff on CD's treated with the Furutech and I've tried every combination of disc there is.
I would compare the effect of the Furutech to going up a step in cartridge from wherever you are now. The overall presentation is less grainy and the highs are smoother. Every person who's ever heard it here (mostly non audiophiles) have heard the same thing, and while they usually don't know how to describe it, they always make the same comments, "there's more music", "the music sounds clearer than before, etc. etc."
This is a fairly expensive addition to anyone's system and I would always suggest addressing room and setup issues (get that coffee table out of the room, that rack of gear and/or TV out from between the speakers!) way before buying one of these. $2000 spent on room treatment will give you a much bigger bang for your buck than a Furutech DeMag.
However, once you have that stuff done and your turntable optimized, I think you will hear enough of a diff using the DeMag to want one if it makes sense for your budget. I think if you hear a proper dem, you will be as impressed as I was.