I like that it's not just another email marketing service - we're able to see who has viewed our content, click through rates etc which helps us make decisions about future campaigns.
It would be great if you could add more customisation options for your templates (eg different sizes of images) but this isn't really an issue as long as they are easy enough to create yourself in Word or similar programmming language. We've found aftersell very useful when trying to target specific groups within our audience.