I have Herbie's TT mat, CD mat, and tube dampener Hal-O's... Every one of those has made a nice improvement.
The TT mat replaced the stock felt mat on my Pro-Ject RM4, very nice improvement in every area.
The CD mat makes for a less digital sounding reproduction, nice.
The Hal-O's tightened the image focus and gave a little better instrument separation.
Every one of those products made a nice change for me, the TT mat being the most dramatic. I wouldn't want to get rid of any of them, though.
-C