There have been a few threads on this - both here are on the adsense forum.
Some people have found adsense to be a waste of time for them. Others, like me, have found it to be extremely positive.
The real answer is that it depends. It depends on what you are selling, to whom you are selling, when those people are most likely to buy something, where they spend their time on the net and of course the appropriateness of your ad copy.
I know it's a bit of an ask, David, but is there any chance you could elaborate on how you are finding AdSense to be particularly positive? Have you found AdSense a good way to get your ads onto specific relevant sites?
For example, www.widget-reviews.com might be a popular site which uses AdSense, and you might sell widgets. So, you find phrases that get your Adwords top on widget-reviews.com's AdSense ads, and you get the best quality traffic?
I imagine honing ads to appear on certain sites is a REAL art! :-)