I've used MailChimp, Constant Contact and Vertical Response for my clients, and prefer CC out of the three for a few reasons. First and foremost, ease of use. CC is much more intuitive. The tools are simpler to understand, the templates are better laid out, and the learning curve is less daunting. Second, there are no limits on how many versions/times you can send test messages to yourself or others (I hate that about about MailChimp). Three, the reports are thorough and the data is easy to understand. And four, their customer service folk are great to work with when you need a real human being.