Pop Up Opt in forms, Any1 got any stats....

I was looking at trying out a pop up opt in form on my blog, however I think they are very annoying, so I was thinking of adding a pop up once the user exits my site to try and capture their email. (not so annoying)

I was wondering if anybody had tested the conversion rates between an 'over the top' pop up or an exit pop up?

I suppose it depends on the niche but I would like to think that the exit pop up is better!!