My bro used to use one in an 8x8 tent with just 1, 1000W unsealed parabolic fixture in it. He had pretty good results with it. I think the main benefit comes from running less light than normal in your room but still distributing it to the plants, keeps things cooler than running the "proper" amount of light. If I were to choose between a light mover or buying more lights and possibly adding some cooling I would choose the latter hands down.
BTW for cooling, I just came across a new design by Climate Right, called the Climate Right 2500 and Climate Right 5000 which are portable AC units meant to cool dog houses, campers and TENTS! The great thing about them is that they sit outside your room and are a sealed and ducted system (which is great for using CO2 as you won't be exhausting any air).