I purchased five of these plugs and began retrofitting some existing instrument cables.
One of the plugs had an immediate failure - it would not mute. I suspect the reed switch itself may have come loose internally or something, but the supplier replaced it at no charge after consulting Neutrik.
A second one failed after not very much use when using it with a Taylor T5 hybrid guitar. This time, it failed in the muted condition. It was not a mechanical issue with the magnet "donut" reported above; it traveled back and forth fine. I suspect, perhaps, that the T5 switches power to the preamp using the typical "short ring to sleeve" method, and that the inrush of DC flowing through the tip-sleeve connnection on the plug probably "welded" the tiny contacts on the microswitch.
Thinking that to be the case, I took the connector and smacked it against a table top a few times to jar the contact loose, and sure enough, it opened up, and the cable has been working normally ever since. I any case, I always carry one or two spare cables (did that before the Silent Plug - it's just good practice.)
The other thing I noticed with the T5 is that when plugging in, there's still a "pop" somtimes, I suspect, due to the actual powering up of the preamp, as if there's a sizeable filter capacitor in the power supply circuit, which might also explain the "contact welding" issue.
I use one of the cables with my electric fiddle, which also has a preamp, and have never had either of these problems.
Bottom line is - I love the plugs, I think they're well-constructed, and they probably work great for passive instruments, and probably most active ones as well, but there may be a handful of devices that, due to the power-on circuitry, will still be problematic. In those cases, it may be possible to change the power circuit to use one of the 1/4" jacks that uses a plunger-operated set of contacts, instead of the common sleeve-to-ring connection, or maybe even incorporate a blocking diode to keep the tip side from passing DC while plugging in.
My $ .02