The best ad integration is done by business.com (of course you have to be a premium publisher) But I agree with !Ask!, adsense on directories (at least general directories) doesn't work.
I bet more than 4000 directories out of total 4683 directories announced in directory announcement forum is using adsense
If you have a free directory then I guess it is not bad as long as you do not over do it, gotta make some money somehow. But directories that mostly have paid listings should not use them.