I've used it when needed (i.e. someone picks that to get started), but honestly Pivotal Tracker is the same thing with more features and even crusty old JIRA has an agile board view option it nowadays where you can drag card versions of the JIRAs around. So Trello is kind of an also-ran that doesn't compare well with the current leader or the industry standard.
JIRA's Kanban board integrates poorly with the rest of JIRA. And you need to define a workflow first. You typically wind up allowing any transition, in which case: what did it get you?
I greatly prefer Tracker, but I would. I work at Pivotal Labs. Everything in Tracker is inspired by how we work and it makes perfect sense in our context.
