Jeanne has a great post up at ClickZ comparing the performance of mail to an opt-in list to performance of mail to an opt-out list.
The article looks at opens, clicks and click through rates over 7 quarters (Q1 – Q4 2010; Q1 – Q3 2011) covering 330 million emails. I strongly suggest anyone interested go read the whole article.
The short version, though, is that the opt-in lists had more opens and more clicks than the opt-out lists. In some quarters it was double the number of opens and clicks.
This data is a strong indication that opt-in lists perform much better than even the best opt-out lists.