I strongly believe that it's really depend from the forum topic, it's name and domain. As if you got domain with keywords, name which exactly reflect main theme of your forum and there are not much offtopics than ads displayed on the forum would be well matched and you'll got good income.
Otherwise if something of this isn't match explicitly to your site or you have dynamic domains which are tricky to crawl for Adsence bot than you'll got a lot of unrelated ads with low CTR.