On the weekend, I spent a good deal of time getting acquainted with Ruby on Rails for the sole purpose of testing out RailsCollab, the ProjectPier-inspired version for Ruby.
When I finally got RailsCollab running, I noticed they have the following features that ProjectPier doesn't have:
-Fully-working calendar on Dashboard and Milestones pages
-Time-tracking
-Wiki
-Message categories
-Task comments (probably using a PP patch)
-Basecamp API (as mentioned somewhere in the PP forums)
-A couple of UI things (mouseover to see edit + trash buttons on tasks, etc.)
Here's a screenshot of a project overview:
http://www.opensourcerails.com/screenshots/1217/original/Picture_2.png
The things RailsCollab doesn't have (and also PP doesn't have) are:
-email notification on task comments
-email notification on milestones
I couldn't find where the Basecamp API is utilized in RailsCollab. Though, I did manually find a Ruby script to do the import. When I tried to run the script, I encountered a lot of errors.
I'd say the main thing PP needs from RC is their calendar system. It's just more robust.