What do you like best?
Mailrelay's free tier is so high, that it's likely that you won't paid for the service until you really grow up - and even then, its pricing is really cheap. Also, it has a ton of learning material and their support service is helpful. Watch out for their blog and their onboarding emails, they deliver a lot of useful tips and tricks. What I like the most about their blog, is that they don't only focus on giving advice on email marketing topics, they also cover digital marketing in general. Speaking more about the app, I think it's important to remark an interesting technology that they have: some time ago, they implemented "Smart Delivery", a feature that sends the emails first to the most active customers, making sure that your IP reputation is the highest possible.
What do you dislike?
Features and design-wise, their competitors are far more advanced. It's UI/UX is really old, and makes navigation a bit hard. And the automation features are quite limited compared on other big players on the market. But I have heard that they are working on this, so we probably will have more advanced marketing features soon.
Recommendations to others considering the product
Take your time to do a full tour of the app to make sure you know all its features.
What business problems are you solving with the product? What benefits have you realized?
The ability to send email marketing campaigns with 0 budget was the main point of using Mailrelay. This is the main reason why I would recommend Mailrelay.