I'm not going to review much of Titanpad. I just think it is an excellent collaborative writing tool. Much of the features has been explained by Lily at her blog http://ezitnew.blogspot.sg/
I prefer it over typewithme as typewithme is very unstable and susceptible to crashing.
Here's an example of students' collaborative work on Persuasive Speech with my input above. This is an excellent tool for process writing as the entire history of student writing is captured under the Timeslider feature.
Here's what I mean by Revisions.
Here are the various pads