Well I would have to disagree with the vast majority. I think that there is some merit to them.
We are currently living in a shed whilst we are waiting for our house to be built (it has been a long wait). We had seen quite a number of snakes around the place, manly blacks last year. The wife had the unfortunate surprise of walking into the bathroom part of our shed to be greeted by a black snake. As far as she was concerned, the living in the shed experience was over and she wanted to rent a house while we are waiting for our own place to be built. Unfortunately renting a place was not part of the plan (the theory that the rent we would have paid, pays for the shed).
So I became gullable. I looked into the snake repellers and decided to get a couple. It was the only way to keep the wife happy. We have had the repellers operating for about twelve months. Although I initially put them in towards the end of the 'snake' season last year, I didn't see any snakes after I installed them. So far this year all the neighbours have had multiple snakes around their homes, I am happy to say (touch wood) that we have not seen a single snake to date.
The theory is that the units send vibrations through the ground similar to that generated by livestock. The snakes are supposedly able to identify that the vibration is a 'dangerous' one and keep clear. Obviously it is still important to remain vigilant, however whilst everyone around us have encountered snakes we have not.......do they work.......jury is still out, if we go through the entire season without seeing a single snake I will probably purchase a couple more to put around the house when it is finally finished.
Proof will be in the pudding