I bought this Streetwize 12v water heating element primarily for draining my golf trolley battery prior to recharging it so as to recover it's full capacity. It seemed a shame to waste all that surplus power simply draining it through the trolley itself as recommended by the trolley supplier, so I put it to good use heating water instead. As such, this water heater serves my purpose very well.
However, I would not recommend buying this for use in the car. It seems to draw as much as 20 amps, which is more than most car cigar lighter sockets would comfortably provide. It also takes an extremely long time to bring a mug of water to boiling point especially in cold conditions when the heat loss from the surface of the liquid seems to just about cancel out the heat supplied by the element. If you want something to warm up a baby's bottle it might be of some use, however since you'll need to run the car engine to avoid draining the battery too much, you'd be better off placing the bottle on top of the engine instead!
Finally, the wire connecting the cigar lighter plug to the heater does get worryingly hot suggesting it is rather too thin to do the job effectively. Also, there is no fuse in the plug itself so if anything goes wrong, it will be the car fuse or wiring that suffers.