I send most of my email newsletters through Constant Contact using one of their templates that I customized to coordinate with my main website. While not the best autoresponder (they just recently added a campaign function), I find their templates easier to use. And… you can send just a text version if you wish. I also use AWeber occasionally, but I find their templates more difficult to work with.
To answer your question, I don’t really think it matters a whole lot if your content is good. However, I think breaking up a newsletter with graphics and white space creates more interest. It certainly does with printed newsletters. (I create and sell both printed and electronic newsletters in my niche.) Hope that helps.