One thing I think people don't appreciate is that Trello isn't really competing with Basecamp/Jira/FogBugz. Those tools are all really well designed for fine-grained project development. Trello is beautifully designed for a much higher-level view of a project. It would be entirely sane to keep track of the general status of your project, while still keeping track of its minutia in something like Basecamp. That's exactly what the Kiln team does: a single card may stand in for all of Kiln HyperSync, even though dozens of FogBugz cases, with due dates and escalation states, may sit behind it. This isn't a failing of either tool; just an acknowledgment they fit into different niches.
That's the post that convinced me to sign up for Trello! I was looking for it to post it, heh. I've been using it for... I guess about 6 months or so... maybe longer, in any case I seem to have the Blue Starfish of being in the first 500 000 users.
If you need more vertical stuff then check out Breeze (http://letsbreeze.com), it's Basecamp and Trello mashup. Plus it adds calendar, time tracking, budgeting and reporting.
We launched BoardTrail, a very cool time tracking app for Trello board, it monitors the duration of cards in the Doing list.
Check it out - http://www.boardtrail.com
And is hilariously expensive, with no free plan. More even than Basecamp.
Pricing is based on the value it's providing. It is not a horizontal tool but deeply vertical. We are not competing with price and also not directly with Basecamp or Trello.