One thing I will add. In the podcast, Jason mentioned that I'd initially lost around 12-15% of Web traffic, since subscribers (and commenters) no longer needed to jump over to the site to comment.

Less traffic, less chance of social sharing. Seeing that, Dylan and Jason have integrated support for several social sharing plugins within the Postmatic email (including the "tweet this" option with Social Warfare). That, along with the increased engagement leading to more in depth comments, has definitely seen my traffic get back to where it was, and often surpass based on certain posts.

This is why I'm such a fan. When they say they want Postmatic to play nice with as many plugins as possible, they mean it. And if it doesn't, they try and find a way to make it happen - and they usually do.