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.