I have often pondered this question?
Here are my own basic thoughts on the complex subject.
Most websites I visit have some kind of adsense system running; I would estimate that some of these are making good money from the Google system. Certainly sites with over 1000 visitors per day should receive a small amount of revenue.
Generally I believe the adsense system is a good one, however like most things these days the adsense system has been misused by individuals...