Jay is basically right. In the winter the hot, high air is brought back to where it does some good by blowing it up, and then it comes down the walls. If it was blowing directly on you (blowing down) then it would tend to cool you through evaporative cooling, even though the air is warmer. In the winter, use fan only JUST fast enough to move the air, not to create a breeze. In the summer, I don't think any such temperature mixing, etc. is the point.
I think in the summer you just blast it down on yourself to cool off. 75 degree moving air will feel MUCH cooler than 75 degree still air. This is because your skin is always a little moist and you get extra evaporative cooling (like blowing on a hot cup of coffee to cool it off). It is really no different than setting up an old fashioned oscilating fan in a room to cool you off, except that it is easier and more effective to have it mounted on the ceiling permanently. The principal is the same, i.e.: being cooled off by a breeze - not any air mixing scheme.
My pet peeve is cheap, weak fans. To do any of this (above) effectively then the fan needs to be large enough, powerful enough, and have an aggressive enough pitch to the blades. Fans that meet these qualifications are typically MUCH more expensive than your typical bargain basement cheapo depot fan and can be quite expensive (like Casablanca brand). You get what you pay for though.