This is a very good plug-in, with lots of features and for a very reasonable low price.
I think that, for the price, you're not going to find anything better on the market.
The plug-in has auto responders and you can manage all your campaigns. All in all, I think it's a very thorough and complete solution.
Possibly, the best newsletter plug-in for WordPress.
It's not perfect, obviously. For instance, I've had some fake subscribers even though I have double sign-up enabled.
Apparently, this is not just a problem with Mailster, as this could also happen with other solutions. So I'm told.
Also, when you're designing your campaigns, the templating system could be better.
But in general terms, you will be very happy with this plug-in. I think it will give you a comprehensive newsletter solution for a very good low price.
In terms of "bang for the buck", it's unbeatable!