Given my experience and the generally very short time i spend being subscibed to a mailing list i do not regard them very highly. The reason for my short time on lists is because they do not provide me with any value, most are little more than "buy this, buy that".
However i am led believe that i must have one. I would like some statistics if possible on mailing lists, like how long do subscribers stay subscribed, conversion rates etc. I'd also expect that some niches are more receptive than others, which ones?