We are using Slack & Trello to manage projects. Each project gets it's own Slack channel. In Trello we have a board that contains Lists for each of the stages in our process. We create a card for each project and move it along the board to different lists as we advance in our workflow.
I have successfully attached the project Trello card to the matching Slack channel by using /trello [url]. I am using the /trello comment command to post information on both Slack & Trello at the same time which is great, but I have found that unless I re-link the card to Slack right before I use the comment command, the bot says it doesn't know which card to comment on. I thought it was a great time saver to just port in Slack and not have to go into Trello for the same purpose, but having to re-mention the card every time doesn't seem to save any time.