If you use the top manual exchanges as rated by review sites like Hit Exchange News, the conversion rate is supposed to be about one signup/sale per 1000 clicks. The positive thing about them is the users are actually seeing your ad. The limitation is 'the clicking community' is limited to people looking for free traffic without paying for it, or using any SEO or linking strategies---i.e., internet biz newbies or folks with no sense of the $ value of their time.

They are folks looking for freebie ways to do things, so if you use a splash page for CPA offer, it might convert well. Or, if you do your own clicking to earn credits, the exchanges could be used to do a rough test of the converting rate of a landing page, in lieu of paying to test the conversions in a live PPC campaign.