I think the biggest consideration should be the physics of the thing. A lot of heat is required to convert hot air into cool air. That heat has to go somewhere -- other than into the room you are trying to cool, as that would result in a net heat gain. I think most portable ACs have a hose that you put out the window or through a hole in the wall (or, perish the thought, into another room). This hose carries the hot air out. If you put the hose out the window, of course, you have to seal up the open part of the window so the hot air doesn't come rushing back in.
The other consideration is that a lot of evaporation must occur in order to cool the air. That water has to go somewhere, too. Much of it probably goes out through the hose, but I think there is also a tank for drippings, which can overflow, if not emptied now and then.
All in all, the only reason to have a PORTABLE airconditioner would be if you want to move it from room to room as the day (or night) goes by.
For my part, even though we have very hot summers, I go with the following method of climate control: windows up at night, to let cool air in; windows and blinds (or curtains) down from early morning throughout the day, to keep the cool air in and prevent hot air from entering and the sun from heating up whatever it touches. Strategically placed fans to blow on you whenever it gets too hot (and to bring in cool air at night), and absolutely no oven cooking. This really works, and I can keep the place at least 10 degrees cooler inside than it is outside for most of the day, so long as I close the windows and blinds very early and don't open them again until the temperature begins to equalize (around 6 or 7 in the evening).
It's hard, at first, to resist the impulse to throw open the windows when it starts to heat up in the house, but this is the last thing you want to do if it is hotter outside than it is inside. Only open the windows when the temperature outside is at or less than the temperature inside.
I've made a science of this, as you can tell. Even though I'm a zealot on the issue, I've made very few converts, so I would love to know if anyone else is willing to experiment with this.