I bought Missinglettr from Appsumo, and when Ben introduced Postbox to Missinglettr users, I bought it and have been using it since its beta release.
Here's how I use it: This year, I wrote a weekly series on Design Awareness. I published on LinkedIn and would use Missinglettr to do a 2-week campaign on LinkedIn and Twitter - it would take me 10 minutes each week to set and forget.
Now with Postbox, I have started making these posts available to other content curators who may want to share my content. I also have access to content from other creators, allowing me to share posts by others related to design. Though there is an automated way to do this, I prefer to do it manually to review the content to make sure it will be useful for my connections / audience. In case you feel some of the content shared by others is not quality content, you can give it a thumbs down. Since my Missinglettr content is already there, it takes me about 5 minutes to share a piece of content in Postbox (I like to tweak the content for facebook, linkedin and twitter) and 5-10 minutes to share other content. In less than 30 mins each week, I schedule posts for a couple of weeks.
I am happy Postbox is on Appsumo, since it will get better with the network effort - the more people who share and are interested in sharing, the better it will be for everyone.
Ben and team have been responsive with Missinglettr and Postbox (during the beta) and I am looking forward to ongoing improvements.