When I browsed through my P2Pool log, I noticed that the P2Pool stale rate varied between 17% and 20%+ in the last couple of days. Therefore the efficiency figure fluctuates even if your own stale / DOA rate doesn't change at all. That is why I think one should watch one's own stale rate more than the efficiency figure.
Both are interesting: your own stale rate varying shows that something changed on your node and help you pinpoint local problems. But you don't want your efficiency to fall too much. Depending on your use of merged-mining (Namecoin essentially), if you fall below the 90-95% range, a centralized pool begins to makes sense if you can't lower your stale rate and wants to maximize your income.
Well, I am merge-mining Namecoin, Devcoin and Ixcoin, so there is some extra there.
And actually my efficiency is over 100% now that the incoming bitcoind connection issue was solved. Now I'm only interested to see how the 0.8.2rc3 update and increased maxblocksize affected the efficiency. We shall see about that this week.