My newsletter is currently double opt in but no one is confirming their subscription.
I think that it is simply too complicated and they do not understand how to do it. The confirmation is in the form of a URL they must visit. Previously I had a reply type confirmation, and maybe 10-20% replied. Now I recently switched software and since then my list has almost stopped growing (even shrunk).
In the week since I switched I've gotten 1100 unconfirmed addresses and I think my overall list size has stayed almost the same. I'm guessing maybe 1 in 50 confirms their subscription.
So I'm thinking about going opt in, my list currently has 13.5k subscribers, and so with opt in I should grow extremely fast, especially in the fall when school starts up again.
Actually yes I just confirmed it, my list has shrunk by 250 members since I switched.