Bloggers are the most obvious example of focusing on this email marketing strategy almost exclusively. SaaS (Software as a Service) businesses are another group that typically heavily emphasize content emails. Of course, bloggers, SaaS businesses, and everyone else can send content emails while also using the other email marketing strategies. And you should do it, too.
If you're doing something right, and it's making an impact, your competitors will pick up on it. If your competitors start changing their own strategies and tactics, and if it seems like they might be doing this in response to your campaign, it's worth looking into. Identify your main competitors and analyse how their own email messaging has changed (if at all) during your campaign.
When you're using email as a marketing tool, you're working on a pretty personal basis. This isn't ‘drive-by' stuff -you're directly targeting customers from their inboxes. There's a huge amount of potential which comes with this kind of marketing... but you'll only reap that potential if you REALLY understand both the marketplace you're operating from, and the audience you're talking to.