I like trello a lot. I'm practically obsessed with it.
It works really well for my small teams and my personal life.
A couple people have said it doesn't scale well to larger teams (with necessary ymmv disclaimers).
I wonder if that's because of how inherently formless and simple it is (which I think is its strength). It's a very general metaphor that narrows to lots of specific use cases easily. It'd be interesting if there were more optional constraints you could add to boards/cards/etc to formalize a specific workflow. I'm just shooting from the hip here, not sure what that would look like.