I've had students use wikispaces to write collaborative papers. It's been successful, but I think primary pad might work for the brainstorming and organization of the writing. I could also see it being used for icebreakers.
I've used Primary Pad in the past for a couple of assignments. I had students work in small groups together to answer literature-based questions. I had another class use it to collaboratively fill in a character chart. I like that I can quickly scan to see who has contributed what to the discussion.
I still prefer Google Docs for most instances, but Primary Pad is fine when you need something quick and easy.
This would work pretty well for my computer science class. We are always writing up some code on one computer that we need to get on another. We usually copy and paste it into email. It would let us have basically a shared clipboard.
I would use this for a number of activities with my literacy groups such as brainstorming emotive words for persuasive writing; creating a never-ending story with each student adding a paragraph using lots of descriptive language to maintain the focus of the reader on the thread of the main story. Students could also collaborate withng a group on a debate topic with the Yes side being given a different document from the No side and then the live debate playing out in class.
I would use this to start a whole class creative story. Instead of passing the paper around the room, students could be adding to the story in real time. I would also use it as a form of a backchannel during lectures and whole class discussions.
I am going to use this with our 5th graders when they come to visit the middle school campus. No account creation means that I don't have to sign them up or in to anything and we can get right to an activity.
It also looks like a great idea for a back channel chat since you can save it later.
"So many worlds, so much to do, so little done, such things to be." - Alfred Lord Tennyson