We've had one on and off for years and generally like them. Yes, they won't keep things ice cold in 90 degree sun but if you can live within their parameters they are great.
We have a narrow Koolatron unit that fits behind the front seat in the car. Plugged into the 12V outlet it is nice to put lunchmeat, fruit, soda, water, etc in it. No dealing with ice, soggy stuff, etc. It works really well.
We will take into the hotel at night using a 120v-12v converter made for it and it will freeze things out overnight. I've had my share of icy sodas in the morning. One thing is after a couple days it can freeze the fins inside and it pretty much stops working. Flip the plug for 5 mins to warm it, melt the ice, clean up and back in business.
In short, not as good as a real fridge but we still love ours.