Give the subscriber a choice. If you're going to distribute an HTML newsletter, make it worthwhile. All too often, HTML e-mails are unattractive or seem to offer no value in being HTML at all. These leave a negative perception of the sender. If you're using an e-letter for marketing purposes rather than simply distributing editorial material, test your response rate to HTML versus text versions before making any conclusions and switching from one format to another.
Preferences seem to correspond to the audience.
For example, an HTML campaign may pull better with a B2B audience than to consumers. Again, testing may yield some surprising results.