Usually people don’t go out and choose one of these units when they could have a window air conditioner instead. It’s only when building regulations or practical concerns mean that they can’t install a window unit that people get these. (Or maybe they just don’t know better.)
The reason why they provide such crappy cooling is just physics. A portable air conditioner uses a hose to vent exhaust out a nearby window, which creates a few problems: for starters, negative air pressure that pulls warm air in from other rooms as the conditioned air gets pulled in to cool the condenser, then vented out the window. With the entire motor inside the room with you, the appliance makes more noise than a window air conditioner. They’re also very heavy, weighing 80-100 pounds, and not all that portable once you set up the window venting mechanism.
Consumer Reports says that if you have no other choice, get the Honeywell MN10CES[WW], which wasn’t a terrible performer, and only costs $400. If you have any choice at all, though, go with a window unit: you’ll get superior cooling for less money.
Are portable air conditioner claims a lot of hot air? [Consumer Reports]