They are very useful, I bought mine at a service station in France a couple of years ago it was about €6. It has 3 outlets individually switched with a red led over each showing which are ON. The important thing to remember is that the total current being drawn should not exceed the fuse rating, usually 10 amps. I run my sat nav from it and charge the moby. The switches are useful because they save pulling the main connector in and out, especially when starting the engine. This is recommended for the sat nav to avoid the surge from the alternator which could damage the device (according to the manufacturer!!).