I manage a site for a company in a fairly small industry where the potential clients aren't always very good searchers.
The site has been using Overture PPC with a $500 a monthly budget for about a year. It is very hard to find decent keywords that convert. We always seem to attract a lot of visitors we don't want mixed in with the visitors we do. I guess as close as I can get to giving a "wigified" example would be that we offer a service to wholesalers but there are many more retail buyers searching for the service; the retail and wholesale keywords are about the same.
So anyways, I suggested trying AdSense to the company because all the ads are targeted at the end-user (or retail buyer). And they are pretty high-dollar ads.
It's only been about 3 weeks since we added AdSense to the site but it looks like we're on target to make up for our Ov spending and maybe make another $100.
So they are getting decent advertising for free.
But like I said, this is a business site offering a service so the big revenue comes from sales. There is also, obviously, the issue of having advertising on a professional site; does it cheapen the site and "turn off" potential clients? This is the first month, so it will take some time to measure any negative effects.
Again, this is an unusual example and we're only talking about $500/month of spending. But I've been wanting to share this example as encouragement in case someone was considering giving it a try...so give it a try.