I'm relatively new to power naps, so I cannot say for sure how much they have benefited me. I do not know about them improving my concentration and focus, but I definitely feel less sluggish after a power nap. With the right amount of time, I find that I can wake up with absolutely no grogginess.
Everytime for a short period a plan for it. I fall asleep and have to postpone my work or sometime i am in problem. It had not worked for me. And also i forget sometimes after it what i had planned to do and much of my time is wasted to remember it again.
It is, of course, a personal thing, whether people gain from power naps. Sadly, I myself get very disoriented, nauseous and tired, with head-aches and queesiness. I think I'm more of a person that needs very regular and long sleep, and stay awake the whole day. Very difficult during exams though, haha