Well my first free promotion ended last night and I'll share my results. Keep in mind, I only have one book and did the promotion for only two days. I got listed on as many sites and newsletters as possible. I reached out to family and friends, and had them share the news on social media. Over two days, I got more than 3,000 downloads of my free book. I even hit #1 and #2 in two top free lists and cracked the top 100 free overall.
So I wake up this morning and I'm pumped to check my stats. And what do I see? One freaking sale. 3,000 downloads produced one sale.
So yeah, I'd venture to say the others in this thread are correct when they say it works best when you have more than one book to offer. Because it sure didn't do anything for my single title.