What does the software or service do well? The price is right, and it's pretty easy to use. I also like their marketing and company culture, especially as I'm also located in Atlanta. I've been using Mailchimp for well over a decade, and there's definitely a reason for it popularity. Their customer service is usually very helpful as well.
What does the software or service not do well? I always have conflicted feelings about Mailchimp. As I said, I like their marketing and culture, and the way they support the Atlanta community. And I've used them at several companies, and a lot of my clients use them as well. They were definitely an early leader, and passed their competition early on. However, in the last few years, I feel like they keep adding more bells and whistles without improving their key offering—email marketing. Paying for the same user on multiple lists, their tagging system, and little things like no metrics on plain-text emails I find annoying, and like they've fallen behind current competitors. I wish they'd improve that before adding in all the new things they've recently implemented. It works for me for now, but I will definitely outgrow the platform for my needs.
What do you wish you had known about the software or service before using? That you can only get Live Chat and customer service email with a paid account. Their knowledge base is ok, but kind of like a black hole, and I don't find it all that helpful. I have a paid account just so I actually have access to customer service.