It is a reasonable solution, OG, which is why many people, me included, use it. However, it isn't a reasonable solution if like in Mikey's case, it proves too costly to run, because the system isn't very efficient (which is why I was addressing my post to Mikey). You say you are unsure what the better alternative is, well it is to use a dual-hose system.
//Replacing hot with cold from the unit and warm from the next room seems a reasonable solution.//
The units don't replace hot with cold and warm from the next room. They create hot air when cooling warm air and expel that hot air outside. If the air comes from an adjacent room as you suggest, rather than from outside, that air is likely to be around the same temperature as the air that you are trying to cool, and in any case has to come from outside at some point. A dual-hose system effectively puts the compressor outside as it draws air from outside to cool it and pushes the hot air back out, which is much more efficient that using air from the room and sucking in more air which then has to be cooled.
It is a bit like running a fridge with the door slightly open - a reasonable solution if you don't have another option, but not very efficient.