First up, just have to say what a great product Opt-in Email is, the best out there in the DNN world from what I've seen. There are a couple of features that we use in a CF email marketing module is the ability to track links.
Links included in the email, contains parameters, and when the user clicks on these links it records it in DB for later reporting. It's very handy from a marketing point-of-view to track the success of an email campaign.
Anyone's thoughts would be great!