I've not tried pair programming using collaborative editor but I've done sessions of requirements gathering with teams using EtherPad (I guess now it would be Google Docs given Etherpad was rolled into Google Docs).
It's fun and almost LAN game like in terms of having everyone create stub items, then others go in and fill them out, then someone else goes in and further adds to that given item, etc etc.
I can't describe how helpful this is for some of the open source projects that I contribute too. Sometimes it's hard enough getting folks to review patches, etc, but getting people to write docs is like pulling teeth at times. This makes it way easier to tweet or announce in IRC that a doc is being edited and in no time you can have 4-5 people working on it and it's finished in a fraction of the time.
BTW, Etherpad was released as an open source project (https://github.com/ether/pad seems to be where the development lives) as part of the acquisition and there are tons of clones out there if you still want to use something simply with no sign-in, etc.
It's fun and almost LAN game like in terms of having everyone create stub items, then others go in and fill them out, then someone else goes in and further adds to that given item, etc etc.