In our company, we primarily used Trello for two different departments: our Brand Development/PR department and our technology department. For Brand Development, we were able to set up boards to help track metrics that essentially our project management tools weren't quite correctly equipped for. Certain opportunities would arise from several different ongoing activities, and it was a seamless visual way to our team to keep track. They were able to, at a glance, instantly see which are the more fruitful activities and allowed them to shift focus to that area.
For our tech department, we used it to collaborate with several different members of our team both in and out of house. The collective feature of the board allowed us to work on projects in sprints. Individuals were able to easily drag and drop items that did not get completed into the next list.
Overall, for a company of creatives this is an amazing visual tool.