I've used Mailster's non-commercial version for more than a year, and recently purchased the Pro version to accommodate a couple of larger lists (over 50 subscribers) and provide some other functionality. Most functionality is identical in both, including everything mentioned in this review.
Options are broad, and seem to expand with each release. You can limit lists to registered site users, or allow visitors to subscribe. You can limit posters to specific individuals or subgroups, or allow every subscriber to post. A large number of message formatting and sending options as well.
As expected in a pre-1.0 release, a few edges are still rough. There's no way at present to send validating or welcoming emails to new subscribers. The only way to catch signup spam comes via a large-format Captcha; that works fine if you use the signup plugin, but doesn't fit well inside the signup module.
But these are small issues, and Holger's ultra-responsive support more than makes up for them. I found a small bug importing a CSV user file from a freestanding Mailman installation, and he had a fix in my Inbox about two hours later. It's not often that you see support like that.
But it's so basic that it's almost impossible not to hit one or more of its limitations almost immediately. It's only possible to have one fixed amount for shipping -- any item, any quantity. Only one possible tax rate. Only one option per item (so if you sell a clothing item, you can't choose both color and size for the same item; you'd need to make each color a separate item.)
Bottom line: it's the right tool only in a very limited, specific shopping setup.
For example, to add an event or comment to a website's shared calendar you have to log in a second time to Google calendar itself, not just the website on which it's wrapped. And as far as I can figure out, it's impossible for group (i.e. website) members to RSVP for an event on the calendar unless they've all been individually invited to attend via Google -- even if they're members of the site.
These issues make it a lot clunkier to use than it otherwise would be. Too bad, because it's a good calendar and a well-done Joomla integration with the modules.