I recently moved to Active Campaign...
And I love it.
Though to be honest the functionality is a little wasted on a newbie like me.
Having previously used Aweber I only wanted to add a little more flexibility to allow me to send people information that was triggered by their behavior.
This basically means that if people open emails, click links and visit content on my site I can 'tag' these actions and send them further information that's better targeted to their interests.
This level of flexibility has seen Active Campaign win over some of the most knowledgeable personalities in the market...
Andre Chaperon (author of Autoresponder Madness) and Shane Melaugh have recently moved to Active Campaign because of it's flexible functionality.