I've successfully used Pivotal Tracker for my last three startups. It's an opinionated tool whose opinions don't always match mine, but it works well, and I've

I've successfully used Pivotal Tracker for my last three startups. It's an opinionated tool whose opinions don't always match mine, but it works well, and I've scaled it to teams up to about 15-20 participants. Beyond that, it seems to get bulky and hard to work with, but I don't know if that's so much a limitation with Pivotal, or with how I've tended to use it.