I have used a free opt-in gift one time before and it worked well so I didn't test much weather it would affect conversion if I changed the content of it.
What I mean is...
Volume (PDF 10 page report vs 50 page report, 10 min audio vs 30 min audio)
Quality ("basically a sales talk" vs "can be sold on it's own")
Format (audio, video, PDF, email "lessons" etc. or a combination of them)
I'm not talking about what would be a good incentive to opt in, but rather what motivates them to actually buy your product or visit the sales page after they receive the gift.
For example, if the quality is too good then they may just get satisfied only by the gift, or if it's too much (audio/pdf is too long, there are many of them etc.) they might stop reading/listening in the middle and forget about it.
Do they not matter much as long as you send newsletters/autoresponder with good copy later etc.?