Easy to setup and monitor
What's not to like? If you are just needing to test drive their service, you can add a contact list of up to 2,000 and send them all 6 emails per month totally free. It's easy to setup and get going.
For small businesses or consultants getting started, you might actually be able to stick with the free plan. It mainly depends on the size of your contact list and possibly features you are looking for. For example, if you want to configure some automation features, you will at least need their "growing business" package.
If you have a really large customer list or email list then prices will start to climb pretty quickly. Their prices are fairly competitive so it's not too expensive. Just take the time to create really nice email templates to get the most from your campaigns.
Not quantified, but I was able to generate a few new customers from a marketing campaign that cost less than one engagement so it was definitely worth the investment.
In fairness, MailChimp is the only standalone email marketing service I have used. I had heard the name a lot and seen it in the footer of some marketing emails I have received. I liked the look of the emails so I tried it out.