I think I've done about ten of them now. In general, I have a fairly low number of impressions for them because I'm not willing to pay a high price per click. I've only had two where someone actually clicked through to the product page, which means that the other eight were essentially free advertising. Future name recognition as I look at it.
Of the ones where people did click through, I spent 4 cents for two clicks on one and had a borrow of a book so made $1.35 on that sale. Just one sale, but a good payoff.
I will say that I see my own ads on another site I frequent (probably thanks to cookies and browsing history) and I personally think they overdo it because the they use the banner ad and the side ad at the same time. But...most people won't even register it, so doubt it will annoy them the way it annoys me.