In general these things are a scam, but they propagate the scam by sprinkling in a few grains of truth, then blowing those out of proportion.
The "Power Conditioner" style energy savers are basing their claims on a few somewhat legitimate issues. One of them is that they "rebalance" the voltages in 3 phase systems, and that saves energy. It's one of those grains of truth, but over blown. The reason it's true is because 3 phase motors will over heat if the votlage is unbalanced, and that heat is energy that's being wasted. But they "rebalance" by just essentially reducing everything to the lowest common demominator, which itself can have other ramifications. All of their best claims are like that, they only tell you the PART of the story that supports their claims. Other stuff they say, such as the reduction of spikes being an energy saver, is just pure bovine excrement. Some of them also put in some caps to correct power factor, which (as discussed here numerous times) makes the AMP meter go down, but does nothning with regards to WATTS, which is what we are billed on. That's the oldest "energy saver" scam on the books.