This is something we have been discussing for a little while. On the one hand, pure-text emails (in this day and age) come across as vague and often untrustworthy by most audiences. The look-n-feel of a richtext/html email wins over most of the time. This is basic UI design. Even more so because the corporate identity can be injected into the email and this already begins the association and decision process. The are complete guidelines available on how to get the most out of email marketing, from using powerful subject lines to the call-to-actions on the email...
One the other hand... and this is a bit controversial considering I have no data to base this on... rich emails can be seen as a type of online advert. We may argue that emails are more targeted, but the reality is MOST of the ads you see are targeted at YOU anyway...
So does online advertising still work? Here are two great articles with somewhat differing viewpoints:
I also found these articles very interesting:
Finally, to tie up, do users respond better to rich emails? Without getting too technical about what a defines a "better response" and etc, the answer is YES. Users respond better to things that are more attractive. Remember that, however, the final experience will be affected by email reader blocking images and etc.