A lot of people bring these back as they are dismayed at the amount of time it takes to boil water.
They often take over 20 minutes to boil half a litre of water.
You need to look at the physics of this. A 12V Kettle will often draw 20 amps at 12V. This equates to 240 watts of power. Whilst this may seem a lot, it is generally only a fraction of a domestic 240V Kettle. They routinely draw between 1800-2200 watts of power. No wonder they are quick!
YOU MAY BE ABLE TO LIVE WITH THEM ONCE YOU ARE AWARE OF THE ABOVE, OR, PERHAPS YOU SHOULD CONSIDER A STOVE-TOP GAS FLAME KETTLE (IF YOU HAVE LPG THAT IS!).
12V Kettles work, but the laws of physics are against them.