They can be fun, informative, even invaluable. Others are disappointing, just like real conversations. After all, no one is in charge of this incredible connectivity. Later, they can just as easily decide not to do it, or be told by their organization that such service is not in their mission statement. Some lists generate a great deal of mail each day, and as long as you belong to the list, the mail just keeps coming.
Many new users find out the hard way that there is such a thing as too much mail.
You should keep the reply from the listserv host that you receive when you subscribe. It will tell you how to unsubscribe. Go ahead and send the message without a subject.