Pros
Mailchimp has a truly professional feel to it. The interface is responsive and well designed. The platform makes it easy to start for free and offers flexible plans, perfect for a startup. Mailchimp has many tutorials available that came in handy more than once. They also use a good sense of humor, making for a less boring experience when reading technical material. There are many options to customize the service and I found it easy to make all the necessary changes in order to set my account for a Spanish audience in Mexico.
Once a campaign has been sent, reports are automatically built. I could even see the stats being updated live just a few minutes after sending everything! There is detailed information about open rates, click rates and statistics about the general industry to see clearly if the audience is being reached as expected. One can also see the reaction of every customer in each campaign: unsubscribes, bounces, number of clicks, etc. All this information is accessible and can be exported to CSV.
Another aspect that I really liked is how well the template editor works. You basically insert "blocks" of text, dividers, images or other types of blocks on the right side of the page and you can see in real-time the template being rendered on the left side. I haven't found any issue when viewing an email sent through Mailchimp based on a template, both on mobile and desktop clients. On the contrary, it made for a consistent design and branding.
Cons
Mailchimp works wonderfully well, but it isn't for the faint of heart when it comes to the initial customization. Sure, the documentation is great, but it does take a while to setup everything. It wasn't clear to me how to use templates inside campaigns, for instance, but even without reading all the documentation, one can deduce much of the functioning by simple trial and error, which ended up being my preferred way to learn my way around.
I didn't find the File Manager as intuitive as the rest of the interface when I started using the product. Sometimes, I would want to resize pictures or apply filters and would get confused as to where the image has been saved or if it were indeed saved. However, it does seem to be getting easier to use in recent versions, as it contains a "revision history" for each uploaded file and an impressive list of adjustments in the image editor, which is a great addition when working on a different computer where the usual software might not be present. I still prefer using external applications when modifying pictures, but Mailchimp does allow to make pretty much any modification you might need and having access to a revision history makes it easy to go back to an earlier version of a file even without first making a backup of it.