EtherPad: worth giving up Google Docs for?
Just pass the URL to your collaborators, and everyone is set to start editing. There are stylistic limitations, with no fancy fonts and formatting, but edits happen in real time. There's also a handy chat box in the sidebar, in case you need to discuss your changes. Where you might want to use Google Docs instead is if you're writing something long that needs to be exported to Word or another file format. EtherPad does saving -- and infinite undos for all users, which is quite handy -- but it doesn't do exporting. You'll have to plug your document's URL in to pick up where you left off.
Complaints? Not many. The aforementioned exporting would be nice, and although it's great that you can quick-start with no accounts, they should at least be an option for privacy. And, speaking of privacy, users can see each other's IP addresses in the sidebar. I'm not sure what that adds. A built-in spellchecker would also be a perk, but it's not necessary. What EtherPad has so far is quite impressive, and I hope it continues to improve, while staying fast and easy to use.