It's dependent on how much sleep you normally get, your individual circadian rhythm, the quality and duration of your nap, whether or not you are prone to sleeping disorders, your biochemistry, and other factors.
There have been times where a five minute nap leaves me completely refreshed and revitalized. Other times, it makes things worse. I'm not certain, but I suspect that the times it has been worse is partially a result of chronic sleep deprivation, such as going a week or two on five hours a night. In those cases, 20 minutes doesn't make an appreciable dent in the deficit.