When I have Power Nap turned on, my Macbook is found in the morning hot and sweaty from trying to run Time Machine (to my Time Capsule) all night long. In addition, I often open it up to find an 'IP Address in use' error on the screen. Since all DHCP leases are given out by my Airport Extreme, and are good for a month I think the chances of this really happening are unlikely. I turned of Power Nap and these problems went away.
I now turn off Power Nap on first boot on any new install / new machine. I do not miss it.
A.