What do you like best?
I actually like their secondary software offerings, BeeFree and BeePro, which allows you to design and build HTML emails and send to clients for review before sending en masse. The email builder is made to work with MailUp primarily, but as an HTML preview it's great to send to clients for approval when designing email campaigns. In addition, once approved, you can upload the HTML to any email marketing program (not just MailUp).
What do you dislike?
For the price point , if you're working with smaller clients, it may not be worth the cost. This is better for agencies or medium to larger companies. The general services provided are excellent, if sending out 1,000 emails a day or more.
Recommendations to others considering the product
If you're wanting to move away from free programs like Mailchimp, you'll find that MailUp has a very nice set of tools, although the learning curve is on the high side. Once mastered, I've been told things get easier. There are still some interfaces that are a bit clunky. I've still not figured out how to upload compressed / zipped files. If you do well with video and FAQ support, you'll be fine. Sometimes it can be a trial to get help from an actual person.
What business problems are you solving with the product? What benefits have you realized?
I found that MailUp's BeePro is great for designing emails and sending to clients for proofing—especially when the client wants you to create an email campaign but doesn't know the platform from which it will be sent (more common than you might realize).