I use just a category of items, and I really define the keywords used. I have had a few sales from the ads, but more importantly you can get great stats by looking through the ad stats. Then you can adjust your listing to mirror the keywords used from the ads. Another thing to keep in mind is that you can stop your ads any time you want. I many times only run an ad for 1 or 2 days, then start up a different ad. By doing this you can really define the ads and get a broader range of ads.