We don't have an extensive email marketing campaign, so MailChimp is used as a periodic touchpoint with account holders - not really has a hard-selling conversion tool. Therefore it's hard to speak to "ROI."
It does give us insight into how well our email campaigns are being received through its basic metrics dashboard, which enables us to identify what our customers are interested in.
The same advice I offered on the previous question - if you need something that doesn't require Mensa eligibility to send an email, and need a short-term, low-cost solution as a stop-gap measure, MailChimp is pretty ideal. Otherwise, if you want something more complex, look into some of the other tools such as ConstantContact or VerticalResponse.
Until we need more, like what marketing automation will give us, MailChimp does a great job for what we need at this time. It's easy to use, has a clean interface, and a reasonable cost that, when you are just using email at a minimum, makes it very attractive for a short-term solution. Plus, it manages opt-outs very well, so you never have to worry about cleaning a list before upload.
It's easy to understand, for the most part, because terms are in common English. However, it's got quirks, and it takes a while to understand the quirks when it comes to using templates, updating templates, etc.