We've had two of these coolers over the last 9 years! They are great!
We lived in Germany and used our cooler all over Europe to help cut down on food expenses and take care of the kids (babies at the time)!
Pre chilling is a must - if you read the instructions carefully, you see the cooler is set to adjust to the temp of what you add. So if you add something at room temp - it will not chill it to 40 degrees. The way around this is to add a container of ice - I put an old washcloth in the bottom to absorb any condensation then add a tupperware/rubbermaid/ziploc container with ice and have no problem with the unit cooling things. One week long camping trip we used the cooler and it kept things frozen for over three days (we had packed it half full of frozen items)!
As long as your car is running you shouldn't have any battery trouble, but other posters are correct - we once drained our battery because we left the cooler plugged into the battery overnight while camping! If you have a weak battery to begin with you may have trouble.
We usually use the cooler all day when we drive, then at night we take it into our hotel room, use the adapter and pull it into a wall socket. Every night and morning I refill the ice container with fresh ice - no problems!
The coolers are a great investment and you'll find all kinds of uses for them. I even use it in the summer and at holidays as an extra fridge for drinks!