CD Mats -- do they work? Are they dangerous?

I'm curious about these. There's a new carbon fiber one that looks interesting (carbon fiber of course having every magical property known to man, and a few we don't even know about yet). Any thoughts on whether or not we should all be using these, whether their snake oil, and how safe it is to use them in our high end CD players and transports?
I use the SID branded one that is green color and have used it in several different CDP's and DVD players with no operational issues. Whether or not they are beneficial is in the ear of the beholder. Like most tweaks, the benefits of these devices are modest, at best, in my opinion and they will certainly not transform your system from an ugly duckling to a swan. If you are otherwise happy with your system and looking for that extra 2-5% that most tweaks are intended to provide, they may be worth trying. Based on my experience I wouldn't pay the price of that carbon fiber one though. I would suggest trying one of the cheaper ones first.
I'm not sure they're all voodoo. There are several companies - Esoteric, Electrocompaniet and maybe others, who feel that clamping the full surface of the disc is a better way to go than just in the center. So, from a mechanical standpoint, the CD mat may accomplish that goal to some degree. I've tried them, but I haven't really spent any time doing A/Bs. I don't think they're "dangerous" in most players, but some players, such as the Esoteric X-05, specifically tells you not to use it, perhaps due to its full-clamping transport. I would also hesitate to use it on some "puck" players, such as Naim, where the extra layer of material may weaken the magnetic bond, causing the puck to fly off and into the machine during play.

Have you tried that carbon fiber one?
No, I haven't tried the carbon one.

I have the Millennium CF and it is worth the price of admission.
I have to agree with Cruz123, I use the Marigo Signiture Refrence with my ARC and yes, it does make a difference, in fact I don't listen to a disc without it. I've had zero problems in use with it, and yes there is not a huge difference but enough of a difference that I'm satisfied that I spent the money for the improvements it provides for me. As with any tweak it is an subjective judgement, if you can get it on a trial basis that would be a way to go for you, also contact your manufacturer and see what tech has to say about it's use as well.
i have the marigo and carbon fiber mat. both reduce focus and have the affect of softening leading edges of transients, slightly reducing apparent resolution in exchange for a somewhat more relaxed presentation.

i prefer the carbon mat to the marigo mat.
Thanks for your responses everyone....guess you have to try for yourself and decide.
I had a De Mat once upon a time. I found it did make a difference - and not a subtle one (relatively speaking, of course).

Bass tightened up and seemed to go a tad lower. A better sense of space around individual instruments/ sounds was provided (better delineation of sounds).

I stopped using it, however, when it jammed my CD player with a particular disc.
I used the herbies black hole and then millennium carbon fiber. I never play a cd anymore without the carbon fiber mat, worth the $ Herbies was an improvement, and the carbon fiber another step up
I too have the Marigo signature 3-d mat and have to admit that I could not tell a difference. Just my experience with it.

I own Millenium carbon fiber mat for almost a year now.I am constantly doing A/B tests.I bought these (i own 2) for ripping my CD collection.It is in my worplace, i listen a lot and i consider myself now familiar to this product.

The differences are not subtle.Whether you like it, it depends on your system and taste.I've ripped 50 Cds this far.At first, i really was surprised to hear the difference it made.I liked it a lot.Most of the changes are beneficial.

Here are my conclusions:
1. It makes everything clearer,more detailed.Focus is much improved.
2. Treble and midrange are more clear, individual voices and instruments are easier to hear.Every melody is easier to follow.
3. What it does in the bass area is debatable.The bass is more clear also, BUT it is less in size,a bit less loud compared to the other parts of spectrum.Individual notes are easier to follow, and you can hear SPACES (small pauses) between the notes.Without the mat,there are no spaces and all the notes seem "connected" more - also less clear.
This effect is similar to "sealed box vs. ported box" effect.
Very noticeable on "Black Velvet" by Alannah Myles.

I will use it on most of my CDs.However, bright ones (with not so pleasant production - Crimson Glory "Transcendence") to me sound better without it.

Is it worth the money?For me, yes.These changes are the same when i use it with my Quad CD-P2, and the mat actually transforms Quad by large margin.Quad has a slower,fuller bass (as mentioned in Stereophile review), so mat improves it drastically.
These are just my experiences, of course.YMMV.