They work real good here in NY, according to one homeowner I visited recently. Used as a backup system, they heat the water to a temp much warmer than the groundwater, especially in the winter. He had one installed 25 years ago and has saved thousands in home heating oil, which was only needed in the very winter. In the fall and spring, thins house used little or no oil at all, unlike most houses, including mine.
I agree that solar cells are a waste of money. But solar hot water heating is a long proven technology.
thins = this hmmm.
Yeah they're OK as backups, but on their own forget it. And outside of semi-industrial I've never needed a backup water heater.