I've been on the free plan for years (note that features at the free level have changed recently) and the functionality available without paying is impressive. There's plenty of criticism for Mailchimp but, considering I've never paid a penny to them for newsletters, automations & audience segmentation, I'm pretty happy.
The user interface changes pretty often so if you email infrequently, it might look different every time you log in. Also, your business doesn't have to get especially large or sophisticated before you'll bump up against some limitations. I don't expect I'll remain a customer, as my business grows.
Someone getting started with an email audience or on a very tight budget, who is fairly comfortable with technology and doesn't mind "hacking around" a bit to get full value from the free plan.