Industry related "opt-in" are better than general.
First the obvious - greater interest potential -- a well written email is well received by someone with a current interest.
The not so obvious - the owner of the list are generally known by the opt-in audience - thus quality control must be maintained by the holder or risk alienating there niche product.
The owner of the list is more knowledgeable about their opt-inners. They have a greater appreciation of the likes & dislikes, where a general mailing list owner really doesn't care (a jake-of-all-trades, master of none sort-of-speak).
Do industry related searchers - I'm sure you will find lots.
The list service I am with is now requiring double opt in...meaning the visitor signs up and must confirm before being added to the list. That way, no one gets signed up that doesn't really want to be there.
If you want a recommendation on a good service, sticky mail me.