I tried one of these on a Rega Planar 25. Wow, more detail, more quiet, just plain better. It's not subtle either, it's very noticable. For the money, it's a good upgrade. Less that $80 I think I paid.
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On my OL Silver I've used the stock (concentric) C/W clamped to the arm stub and Twl's DIY suspended C/W, which hangs a weight from the stub on a silk or nylon thread.
The suspended C/W obviously decouples the mass of the weight from armtube resonances nearly 100%. The sonic benefits were increased clarity/reduced bloat, particularly on the trailing edges of all notes at all frequencies. Removing that big hunk of mass from the resonating armtube clearly fed less energy back to the cartridge end of the arm.
I had both the Express Machining and the Kerry Audio counterweight at one time, I kept the Kerry F2. I tried both on a Rega P3 before I got rid of it and the Heavyweight. The listening test was b/w a RB300 w/ the EXpress Machining Heavyweight vs. the RB 250 WITH Express Machining 2xtreme endstub with a Kerry Audio counterweight. Do not take this as a TRUE test since they were on diff. arms(another debate entirely) I felt and heard more extended highs with the 250 arm, though. Now, was this the endstub, counterweight or the 250 arm(lack of spring) who knows? I hope this muddies up the issue for you.;)-~ The F2 from Kerry does have ridges that the counterweight rides on while attached to the counterweight, tightened by a plastic? screw down bolt, instead of metal(at least where it contacts the endstub). This may help with the resonance issue?