Hm, at first this is not a bad idea.

As opposed to spam or other Twitter ad-networks where you need thousands of followers before you can earn a dollar, this creates a win-win situation for both the payer and the payee. Instead of hoping that people who downloaded your free product will recommend it to others, you can now rely on it.

However, you tweet before you have seen the product. Thus you might be recommending crap to your followers that you would rather warn them about.