A great product if you want to have the same users for your mailing lists as well as your Joomla website. As a non-profit-organization, we need to offer services to our members (through various other Joomla components) as well as maintain mailing lists. Thanks to Mailster we now can do all of this though our Joomla website.
The current version is much improved compared to older versions, especially in a Mac-and-PC-Environment. The admin interface takes a little time to get used to, but once you get it, its really easy.
But the highlight is the support! Fast response time, extremely knowledgable and approachable developers!