I've traced pages where my ads aren't showing: Pages where my client's competitor is paying $$$ for a banner ad. Guess where my client's spiffy new AdWords ad is showing now?
On the other side- There are other ads that I haven't traced yet. But I'm working on it.
I suggest go back to first principles and ask yourself whether your ad would be work out well in the type of sites that come up on the left side. OK, the people who go to these sites may include more enthusiasts, more people who have more than just a passing or one-off interest in your product. They probably are not as "motivated" to buy straight off, but they are possibly more "qualified". They are not actually "searching", but are more lkely to be informed.
My theory in use is that for specialist, nice products, especially b-to-b and for which you want to reach a qualified niche, content sites may well provide better ROI, (and some of our tests are proving that - we are getting less CTR, but people who DO click convert better and easier)
For broad based consumer items this may not be the case but if you are selling products and services to targeted niches, my feeling is that content sites may actually perform better.
There is a lot of "spin" going on by people "assuming" that content clicks are less valuable, and some people post in the Adwords forms blaming Adsense for a wide variety of problems. On at least 2 occassions is has taken those who initially posted the problem to let the poster know it had nothing to do with Adsense.
In many ways, it is far easier to fraudulently click on competitors you dont like in google serps, rather than Adsense pages. The former provides a much better way of finding your competitors!
So to me, i say think about who your customers are and why they click, and do a few tests with separae campaigns and following up on where leads come from as part of your testing.