If you market to small or mid-sized businesses, you may be interested in this little study from Bredin Business Information:

In the study, SMB executives were asked to judge the relevant
importance of different media for business management information. 83%
of respondents indicated that email newsletters were either very
important or important sources, putting it in a near statistical tie
with print media (84%) and ahead of corporate/media websites (71%). At
the bottom of the list were many of the "new" media distribution
methods, including webcasts/podcasts (40%), RSS feeds (39%) and
blogs/wikis (34%).

They also included the kind of content that small businesses say they’re most interested in (ideas for your own email newsletters):

By far, practical "how to" information (40%) was the preferred form of
content, followed by company product information (26%), management
overviews on topics such as strategy or leadership (21%), company news
(19%) and case studies (17%).

Other interesting stats are in their report, such as how often SMBs prefer to receive your newsletters, what day and time they prefer to receive them, and more.

http://www.bbionline.com/press_0307.htm