Forum Moderators: skibum
I think the reason it is so valuable is because people are actually requesting information about your subject on a regular basis.
Some things to look for:
-Make sure the newsletter requires people to confirm their signups. This eliminates dummy email addresses and people who don't really want the emails.
-Make sure the newsletter is distributed in both HTML and Text and that your ad will appear in both.
... thats all I can think of.
Conclusion - Newsletter advertising works for me.