Unfortunately I must admit that a nicely formatted HTML email looks rather better and more catchy as a newsletter than a simple string of plain text. This goes against my general email preference, but even I have relented in recent times and started allowing my mail client to display HTML email (although I still never send it)!
The Grails mail plugin is an invaluable addition to most Grails based sites - it's a rare website that doesn't need to send email. Some really useful features have been sitting on it's todo list for a while, but it still does a lot as it is.