What does MailChimp do well? Mailchimp allows you to create newsletters and manage listservs. It does so at an incredible price point as well.
What does MailChimp not do well? It is such a finicky and glitchy system where the fonts and "blocks" don't always do what they're supposed to. It requires more work than needed.
What do you wish you had known about MailChimp before using? If all you need is a simple way to manage audiences and send communications, then this will work. If you need any sort of design beyond that, there are better options.
Have you used a competitor to MailChimp? If so, how does it compare? Yes, I've used Constant Contact. Although it's not perfect at all, it's a huge step up from MailChimp.