I can sort of answer this... as before I found OpenGoo I was (trying) to use Project Pier for a client collaboration setting....
OG has a much better interface.. bar none.
PP had a basis of projects, inside those projects you could have notes (with replies), tasks, milestones, and files. You could also cross link any of these items, like tie a task to a milestone or a note to a file.
They share the ability to assign tasks, notify participants, etc.
The areas OG is moving well beyond PP:
Workspaces, projects whatever you want to call them, the ability to have nested projects/workspaces is huge.
Having contacts is great... a calendar... and granting access to workspaces on an individual basis is very easy to do. And of course the ability to not just post files, but actually create/edit... this will really set OG apart as it continues to improve.
One thing I like in PP that is not available in OG is the ability to mark items as "private", which allowed only owner company members to see items in a project, but not the client members. You can sort of work around this (and I do) by having two separate workspaces. One shared with the client, another not.
I will admit, we never got fully implemented with PP as a tool for our clients, but after just a couple hours of playing around with OG, I knew I wanted to replace PP with it.