One point that nobody has mentioned is that there are several types of portable cooler.
The simplest and cheapest type is an evaporative cooler which simply blows air over a surface which is kept moist. The heat needed to evaporate the water is taken from the air, which cools as a result. The downside is that the air humidity rises. This is fine on a hot, dry day, but not what you need on a humid day - and in fact it would have trouble providing any cooling on such a day.
"proper" portable air conditioners come in two varieties:
A single unit combines the evaporator and condenser coils in the same case. The waste heat is disposed of by passing some of the air from the room over the condenser coils and expelling that through a pipe to the outside. The problem with this is that warm air has to be drawn into the room to replace the air lost. For this reason, this type of unit is more suitable as a spot cooler. It may make you a lot more comfortable if you are sitting in front of it, but its ability to reduce the overall temperature and humidity of a room is limited.
A split unit has the condenser coils and a fan in a seperate unit outside, and the refrigerant is fed to it via a pipe. This means that no air from the room is lost expelling waste heat, so these units are much better at cooling and dehumidifying an enclosed space.
Edit: A single unit could be designed to draw air in from outside via the pipe. I'd be interested to know if any of them do in fact work that way. I've always wondered why the ones I've seen weren't!