When I first added AdSense ads to this blog, I had no idea what to expect. After three years, I would have to say that the results have been mixed, but overall I've found that AdSense is worth the effort, and to my mind, actually adds to the usefulness of the site.
For the first few months, AdSense ads earned only a few cents a day, with occasional spikes up to one or two dollars per day. Daily earnings slowly increased to an average of about $1.50, with occasional spikes to five dollars. That was enough to cover the cost of all hosting fees for all my sites, with some left over for new software and subscriptions.

With recent improvements in the layout and structure of my blog, ad performance has increased along with readership of the blog. Earnings continue to rise very slowly, but will probably never get much beyond hosting-fee money.

Google does make it difficult to determine which ads pay the most, which ads actually got clicked on, etc. but it is not hard to determine which content pages receive the most views and clicks. Google usually puts elearning software ads on my site's home page, and most of the clicks come from that page these days.

As a result, I assume that a lot of people come looking for information about elearning, which in turn has led me to post more tutorials and templates relevant to that audience. I actually find the google ads to be useful, myself. I can't click on them, but I've learned about products I wouldn't have found otherwise from them.

It's hard to say precisely what causes increases in clicks. Visitor traffic has gone up somewhat over the last three years, and spikes definitely occur when an article is mentioned on a popular site, like digg or adobe exchange, but spikes in AdSense earnings are often unrelated to traffic increases. It just seems that sometimes very high-paying ads simply decide to appear on my site, probably because of luck or some mysterious google algorithm.

One negative thing that does occur with great regularity is that earnings will slowly ramp up to a higher level over several weeks in a row, then suddenly drop back to a few cents a day, even if the traffic on the site remains the same. Perhaps Google runs out of high-paying ads, or perhaps particular ad campaigns come to an end.

Even with these drops, the ads still average enough to pay for all my internet experiments and hosting. Within the AdSense program there are various types of ads to choose from: text, image, video, referrals, search. The only ones that I've found to be worth using on this site are text and image. I use the search feature because it adds value to the site, although it earns nothing at all. The video and referrals don't add any value and don't earn anything, so I removed them soon after first trying them.

Optimizing is a big topic on the Google AdSense forums, but beyond a certain point, the types of positioning recommended for higher earnings detract from the usefulness of the site, which I am not willing to do.

In contrast to my generally positive experience with AdSense, Amazon ads earn almost nothing. I keep them on the site mainly because I think it is useful to refer people to books on relevant topics to what they are searching for, whether or not they click the ads. I also like to advertise books that I've found interesting, "just because". Other than that, I am not putting any time into "optimizing" for Amazon ads.

Google's AdSense program is probably one of the few affiliate programs which do pay for the content publisher to include on their sites. Although AdSense has encouraged the proliferation of spam "made for AdSense" sites, I think it has created an incentive for people all over the world to post more useful information on the web.