Better than no cool box at all. At least you can put ice packs in it and then plug it in when in the marina or under engine. However remember they tend to draw at least 4 amps and run constantly irrespective of the temperature inside.
We bought one from Halfords for about £50. Uses 3 amps, and has managed to freeze things. Am installing a dedicated solar panel to service it this summer, and that bumps the bill up somewhat!
At a rough guess a dedicated solar panel would mean the cost is much the same as putting in a Waeco fridge unit. We now have a built in Waeco unit and it is wonderful - but I'm still going to go to a 3 battery set up :-
engine start
instruments
fridge
We do find a real fridge is great for the obvious things like milk in the morning tea and keeping meat till the next day.
do be aware that the fans on fridge heat dump coils can keep you awake at night
Uses quite a lot of juice and only keeps the contents about 10 to 15 degrees below ambient. But if, like me, you have no fridge the certainly better than nothing.
I have got one.Use it at home with the mains adapter to get it cold.Put frozen milk in it.12volt in the car,transfer to boat.Only use when engine is running as it draws 4 amps.Works for me.
There were some tests in Dutch magazines:
They use 3 to 4 amp but don't work was the outcome.
I decided to do it like this. I use a cool box which will be cooled down by ice packs before leaving home. When the cold drinks, meat etc goes in, I put 4 new large ice packs in. For weekend use it's ok. No electricity used! After two day's the bear is still cold. (In fact, the first day; your bear can be frozen!)
Bought one of the not so cheap ones a couple of years ago - draws 5~6 amps, cools to 20deg below ambient. switch it off at night. Like ang fridge if you keep opening it it will warm up quicker than left closed. Cools a couple of dozen beers in about 1 hour. During a 3 week cruise we never lost any food or milk even after 5 days between stops and no flat batteries (Sterling controller).
A word of advice, keep it in the shade and well ventilated
Other advantage is you can take it with you when shopping as a cool box. The model I bought had two lids - one coolbox and one with the cooler mounted.