I does cool it down actually, but I wouldn't recommend it for a long term solution as it will increase dust build up inside the PC. I actually had an overheating problem with my old PC, and I tried that and it stopped all the problems I had.
will definitely cool it off, but you
1) have an open case full of very breakable components (spills, kids, etc)
2) may or may not have dust buildup (if the fan blows enough, the dust usually leaves the case, but, pockets of air that aren't moving (say the fan doesn't blow the bottom of the case well) will gain alot of dust