At my last place we used Trello from the day it came out (and had used fogbugz for years). At my current place we use jira + agile. I do prefer Trello for high level stuff, but I think jira does a good job of handling both high level and low level tracking in the same app. Their agile stuff seems to be getting better too.

I'm glad to see Trello is going so well. It's a great product and a great example of not having to build something that covers every possible use case.