I never found anywhere on the site that it supports HTML newsletters.. I read that what they're offering are merely autoresponders for people who'd like to sign up for your campaign..

One of the free newsletter engines I kinda know (but haven't tried is PHPList)..

The paid ones so far are still the best if you ask me, they eliminate most of the important problems you'd normally face when having to send newsletters.

If you can invest in it, have a look at:
www.campaign-monitor.com
www.mailchimp.com