Yes surge protection systems work. Different brands and products give different results. Generally speaking you will get what you pay for. I started out with a simple $100 unit and upgraded to Progressive Industries $300 EMS. Will they stop damage in a direct lightning strike? Of course not. Taking a direct hit will generate problems far beyond frying your AC and other toys. :S
In the last three years I have encountered a dozen bad pedestals {all of which were detected by my protection units}, any one of which could have caused me serious grief. Will a simple surge protector tell you if the pedestal has a fault {bad ground, neutral etc.}, absolutely. You can then move to another site or get repairs made to the one you are in.
Will and EMS shut down the power if the voltage goes too high or low, YES! imagine a crowded, older campground/RV Park on a hot afternoon. Everybody cranks up their AC at the same time and the voltage throughout the park/CG drops dramatically. Those with protection suffer no damage. Those without end up buying new compressors or worse.
That feature alone makes the price of an EMS money well spent vs repairing/replacing your AC, grig, TV, stereo, microwave etc..
For those that think they have no value... you are wrong! "Penny wise and pound foolish" comes to mind.
As always.... Opinions and YMMV.
:R