I am using one now. They're pretty good. Quiet operation and does a good job picking up heat from my video card and dumping out of the case through the vents.
I don't have a way of monitoring my temps now on my current system, but once I get my new PC setup, I will install the temperature software and see how cool it will run. For now, my new case only has 5 fans in there and nothing else.
I would use these fans. They push a lot of air out and not too loud at the same time:
Vantec 80mm Blue Spectrum UV fan