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.
One problem with reaching conclusions on the data is that one needs quite a high hashrate in order to get a narrow confidence interval on the stale rate in a timeframe of a day or so.
For example, my mining rate is about 4.5 GH/s, and the stale rate interval reported by P2Pool is -+5% of the real stale rate after mining with the pool for three days or so.
Yep, mining with less than 10GH/s makes things difficult. I'm at ~9GH/s and I don't fully trust my stats. I often double-check configuration changes when differences are low enough to be explained by variance in my share/stale rate.