I was a long time user of Constant Contact until they treated me like a criminal. My crime was that I hadn't sent an email in ages to my email group and during the economic downturn many of my contacts had lost their jobs, or had changed jobs, thus losing their email accounts. Too many bounced email was my crime. I had paid for Constant Contact for literally years.
Along came MailChimp and it was easy to use, had a friendly interface and was just nicer to use. Templates are more contemporary and they just seem like a hipper company. To subscribe and unsubscribe was so much easier for me as well as people on my mailing list. As the owner of an emailing list, you can divide up your subscribers into separate groups. I love that there is a no cost to use option. I would definitely recommend MailChimp for anyone needing an email campaign designer.