Over on Kindleboards, they do a lot of sharing of sales figures, which is very helpful. I had terrific results from my Kindle Nation Daily (it more than paid for itself, which is what I want an ad to do), but I'm not seeing as good results reported from ads on other sites. I'm not sure KND ads are doing as well as they were, either-- could be the novelty has worn off, or it could just depend on the book. Of course you can gain a lot from an ad that isn't obvious. Exposure over the long term is valuable, and specific sales for the day or week the ad is live don't always tell the whole story.
Nevertheless, if I buy an expensive ad, I personally want to see a significant bump in sales that more or less pays for the ad. If you're thinking of purchasing an ad, I would suggest watching the books that use that space for a while, checking the rankings of each book, and seeing how many of the books make it into the top thousand or the top five hundred. Top hundred in any given category doesn't mean much-- being in the top hundred of, say, contemporary romance isn't going to pay for an ad. You only start selling a lot of copies once you start moving up through the top thousand books in the entire Kindle store.