In Seth Godin's newsletter today, he advised marketers and media buyers to ask email newsletter publishers what their open rate was, before buying an ad. (An open rate is the percent of people who open an issue, instead of deleting it from their in-box. Only HTML newsletters can track this. Text-only can't.) In fact average open rates should be included in the media kit, but they almost never are. An open rate of 50% or above is highly respectable. The 50% won't always be the same people -- you know, sometimes you read your newsletter issues, and sometimes you don't have time, or the headline doesn't appeal to you. So if you advertise in a newsletter repeatedly, over time you'll reach a larger portion of the readership than the average open rate as different people read different issues.

I would give you a link to Seth's newsletter here, but he doesn't post it online. (He emailed me that he's working on it.) You can subscribe to his free newsletter here.