Unfortunately, they suffer from some heat leakage. I have seen a claim that you can reduce this by insulating the flexible duct that goes out the window, but that is not the complete answer. I have looked at some in shops, and if you feel around it, you can always find a warm corner or two on it. So I wonder about the ratio of (cooler power)/(electric power in) how does this typically compare to a (fixed) split-system?
I know there exist some portable split systems - delonghi and one of the Jap companies make them, but they seem to be pretty rare.