It's very easy to use, has good customer service and support, is not expensive for small or large companies with limited resources, it also allows you to have multiple campaigns at once which helps when there are more than one person in charge of different projects.
I haven't found anything bad about this software so far but maybe because we don't need all its features yet. We're using it as an email list manager, it works well with our CRM system and is helping us improve the way we communicate with our clients.