(I guess I see a lot of ads for www.AreYouASlackerMom.com, too, but I completely ignore those since the answer to the question, for me, is clearly "no".)
I clicked on the ad once, a few months ago. It had a bit of a hook—"A shocking secret coffee co's don't want you to know"—and after I'd seen it a dozen times, I decided I wanted to know what the secret was. (Hint: it's not particularly earth-shattering.) I looked at the site and decided that I have absolutely no interest in it: they sell and ship overpriced coffee beans, and if I am going to buy overpriced coffee beans I'm just going to buy them someplace nearby.
But the ad did its job: it piqued my interest and I clicked on it. I looked around and evaluated the product. The problem is that I made my decision not to buy anything and I'm not changing my mind—but I still see the ad every day. The ad impressions are being wasted on me, and would be better targeted to someone else. In fact, I would argue that, at this point, the ads would be better targeted towards someone who has never mentioned coffee in an email than to me, someone who has seen the ad 60 times, clicked on the ad after 12 viewings, and never returned.
Part of the issue may be that they aren't measuring/using optimal frequency: it's good to show people the same ad several times, but after a certain point response rates drop. I do think that the optimal frequency is higher for a text ad than it would be for a splashy ad graphic, so maybe showing it to me 12 times makes sense. But if they are using some kind of target frequency, maybe I screwed it up by clicking on the ad, thus expressing interest in it?
That's really the conundrum. To improve keyword ad targeting in a service like GMail (which is different, I think, than keyword ad targeting on Google's search, where you're not likely to see the same ads repeatedly unless you search for the same things all day), I would suggest not showing an ad to someone again after they've clicked on it. But maybe that's a bad idea—maybe those are just the people you want to show the ad to again, because they clearly have more interest than the people who have never clicked?
Potentially, the first click would signal interest, at which point they should start the switch up the ad copy: promote specific products, sales, etc. (After all, I'm already familiar with their site, so the "shocking secret" message isn't effective anymore.) Who is more likely to become a customer, someone who has visited your store before, or a new visitor?
I don't know what the answer is, but I know I'm not buying any coffee.
I e-mailed this text to CoffeeFool.com, and here is their reply:
We get lots of mail from those who see our ads. Here's one particular email that might answer your question:
"I am so impressed how you stay on top of the Google ads every day.
If you keep this up I will buy some coffee...Guaranteed"
All I can say is, the ads on GMail must be cheaper than dirt.