Our company started using Pivotal Tracker to manage software development in an outsourced environment. It seems to be a really successful tool in that regard. It's collaboration features are strong, its assignment tracking is very strong, and its reports are pretty good. If you have an application to manage distributed tasks that are each of significant effort -- say a few hours to a few days each -- then Pivotal is a top contender and you should review it carefully.
The problem that I have with Pivotal is that its success in the coding side of the business led to its adoption across the board in all our business processes. Here's where the trouble started. It's a little too slow to use with tasks that should be measured in minutes rather than hours -- it adds too much relative overhead. Of course, there are ways to deal with this like using Zapier to integrate a list tool or Trell or the CRM with Pivotal... but then the real question is why go to all these lengths to apprise teams in other departments to a level of detail they are not interested in?
So, if you're developing code, debugging code, or create documents that that, say, 2 or more hours each, Pivotal Tracker is an excellent way to organize, and, well, 'track', progress, report on relative team velocity and, most importantly adapt workload to current work pave (it automatically moves stories from background to active based on the calculated velocity/workload of the team -- that's pretty cool.)
But if you just need a list, or if you existing tool, like a CRM, already has task management then don't complicate your life.
Value for money
Ease of use