I love the ruby library for mailgun. It makes it super simple to get transactional emails up and running on my Rails projects. Also the REST API is a cinch to work with.
There isn't much to say about the downsides of mailgun. The UI is probably a little less polished then some of the competition, but I don't have to interact with it much on that level, so I don't notice.
If you work with Ruby on Rails, mailgun_rails (https://github.com/jorgemanrubia/mailgun_rails) is a fantastic way to get up and running with transactional emails quickly and easily.
Building applications for the web. Using Mailgun (via the great Ruby library) means that I can't get emails up and running quickly for my clients, while still providing robust extensibility and customization when needed.