While I have not surveyed subscribers, I believe that they are mainly involved in the practice of law or legal research. Net-Lawyers not only has great discussion be a wide variety of people, it also has a good reputation as a list without much noise. Sponsoring the list reaches professionals with purchasing power, and I believe it has the possibility of creating a substantial number of visits to a web site. The list is moderated, which means that I, as moderator, make sure that nothing inappropriate or off-topic gets distributed. I also keep the volume of the list at a manageable level.
Sponsorships are available, such to prior commitments, on either a weekly or monthly basis.
If I don't have a sponsor for a particular week, I intend to announce that on the list and offer the sponsorship to the highest bidder, if any. Please note that I intend to approve posts on a daily basis. However, it is possible that due to a crunch caused by my law practice or for other reasons, I may not be able to approve posts on some days.