They work.
You can buy CteK at most all exotic car dealerships, Porche, Lambo, Mercs are driven intermittently compared to other cars and so these battery groomer/maintainers are designed for constant connection.
I have used smart battery chargers/maintainers for years now and never had a problem (NOCO prefers to operate above 0C where CTEK goes to -20C) Go on any forum and the resounding opinion will be the same. It keeps the battery charged year after year with no issues. I have left mine connected to cars, ATV batteries for months on end, each and every time I go to start the battery behaves as if new. If it is good enough for a $200,000 or $500,000+ car, its good enough for my DD.