I came to loathe the columnar setup of Pivotal. I found myself constantly rearranging the interface just to be able to work.
We have a lot of project and subprojects here, and Planbox's hierarchy (there is basically one more level than in Tracker) is really useful. Their UX for slicing and dicing based on person, label, project, etc is also nice. The search is faster (it's all client side so it's practically instant). The charts make more sense to me. It doesn't treat stories as a unit of work; rather they are a goal. Thus all of the tasks in Planbox can be individually assigned to different people and estimated and time-tracked at that same level of granularity.
Overall I just found that there were a lot of things I was hacking around in Pivotal, and frankly I didn't even notice it. It's only when we hired a couple of people and they pointed out frustrations with Pivotal that I started looking around. When I started using Planbox, that's when I really realized how much I was adjusting myself to the limitations of Pivotal. I find myself much less stressed managing things via Planbox.
Hope that helps!