hpvic - not sure if you're working this solo, or as a bigger team, but I'm a PM, I love Trello, haven't used Pivotal, and can't stand Jira. I've been considering building out a Jira-killer myself as a side project. Might be good to chat and hear more of what you're working on. If you want to connect my email is cameron.norgate at the gmail. Cheers!
Pivotal works but lacks the idea of "stages", which is very useful because it actually reflects the real world.
Trello is cool but it isn't made specifically for software. For example, it doesn't have acceptance workflow.
Combining ideas from both of these results in a very compelling and (I think) useful product. Version 1 will be ready next week.