I'm a pretty big fan of Herbies Audio Lab stuff. Admittedly, not every Herbie product has worked as well as others and some seem to be component dependent. But, some of Herbies products made dramatic differences.
Herbies products that I liked: 1) Iso Cups. Really liked them with the standard hardballs at first, but then went crazy and tried one of the Gabon Gyros which were expensive at $35 a pop. I tried only one in the beginning and liked it so much that I had to have two more to put under my amp. No single tweak that I've tried increased the holography and 3-d imaging as much as those darn Gabon Gryos. I tried taking them out once and put the standard balls back in. Ugh. I couldn't live without those Gyros. The use of the Gabon does tend to emphasize the high end a bit, so its not the best match for every system (what is?). Herbie is offering some new Gyro ball, but I haven't tried them so I have no idea how they compare. 2) The Tenderfeet. I'm more of a fan of his older smaller footers than the newer ones, but the newer ones worked well under my turntable. 3) Sonic Stabilizers. I was amazed that these did anything at all, but they do. Very component and placement dependent. They seemed to not only do what many isolation devices do, ie., add clarity and sharpness, but they also altered the tone by making it fuller and richer. 4) The Hal O tube dampers. Also depends on the model/material that I've used and where its used. Adding the dampers to the tubes in my phonostage yielded tremendous results. Also got great results on the input/driver tubes on my amp. With most other places, the results weren't as noticeable, but there was nothing negative either.
I didn't care for the Herbies record clamp/stablilzer on my Rega turntable. Made the sound muddy and messy. That well could be a Rega issue as there seems to be a consensus that Regas don't fair well with clamps. Fortunately, the clamp can also be used exactly like the Sonic Stabilizers so it ended up being a gain, just not for my turntable.
The CD mats, including the Black Hole, was the weirdest. They really do work, and I don't know how or why. Quite subjective as to whether using them is an improvement or not. I loved the mats at first, but later decided that the sound wasn't my cup of tea. Hard to explain why, but I felt I was losing some dynamics and high frequencies. Others have liked them and continue to use them so this well could be system matching or personal taste.
Hate to sound like an ad for Herbies products, but his 90 day return policy is so great and hassle free that you can't go wrong trying anything he sells. The worst that can happen is you lose a couple of bucks for the shipping. Plus, Steve is just a super nice guy and generally gives sound recommendations and advice if needed.
Anyway, I thought this was going to be a short post of me just saying that I liked the Herbies tweaks. Man, am I long winded.