Monster power conditioners use MOVs, which have a relatively limited lifespan. After the MOVs are gone MANY times the units continue to pass current to the equipment meaning that the next surge could destroy the connected gear. The Monster units do have a light to indicate that they are no longer protecting the equipment but who wants to depend on a light that they can forget to check?
Panamax uses an avalanche diode that acts almost like a flood gate. It opens when voltage is above or below a safe area, then starts testing the voltage. After the voltage is back within the normal range for 7 seconds Panamax units allow the equipment to receive electricity again. When an avalanche diodes fails they are stuck in the "open" position 99.9% of the time. Meaning that the equipment receives NO electricity so the user knows that the power conditioner needs to be replaced.
Panamax specifies that MOVs are 1000 times slower then their avalanche diodes. At trainings Panamax regularly does a demonstration where they hook up a Panamax and a Monster into a device that can create spikes, then into each unit they plug an illuminated lamp. Both units are then hit with a 150 volt spike. In the 10 or so demos that I’ve attended I’ve noticed two things. First before clamping down the lamp plugged into the Monster grows momentarily brighter. A sign that is letting the beginning of the surge through before clamping down. Hence the potencial for damage to connected equipment. No increase in the brightness of the lamp plugged into the Panamax is observable. Second, I’ve NEVER seen a Monster live past the third “hit”. Twice they began to smoke, once a small fire began. The Panamax products have lived through 10-15 hits before they go on with the presentation, they don’t fail mind you the presentation just moves on because they’ve proved their point.
From what Panamax says other surge devices using MOVs will act in a similar manner. If you are considering things up market from the Panamax products Transparent Audio’s power conditioners also use avalanche diodes and offer some of the best power conditioning that I’ve ever heard. PS audio’s new power conditioners use something in addition to an MOV. They claim that these two devices in tandem are better than an avalanche diode. They may be, for me I look at how these devices work in the long term (ie years) and their products are just too new for there to be a body of empirical evidence out there, one way or the other.
For my own use I've used the original version of Panamax's MAX-5510 for about 4 years in my high-end two channel system. For my computer, TV, and AV receiver I use one of their cheaper units.