Hey there, I hope you don't mind me chiming in.
I actually use Google Docs for a lot of off-site writing. I'm not sure whether or not all parties have to have a gmail, but it doesn't take long to just make an account. Usually, you make a doc and then there's a sharing feature where you input the recipients' email addresses, so I'm assuming others will work.
As far as the system goes, I find it's really handy. In terms of posting speed, I'd place it in between IM RPs and board RPs. If you're looking to RP on it, my partner and I usually use two different colors for writing. I'll write in a maroon and he'll write in black. We usually put some symbol at the end our "posts" or paragraphs to signal that we're done writing and the other can respond. A pound sign, asterisk, or something. I know there are other features, where you can insert a drawing if you wanted to do some mapmaking, or one where you get email notifications if a comment has been left. I suppose the latter could act like notifications that you'd get here on E when someone replies to a thread.
I've used it a couple times on my iPhone, but I find the process still to be a little finicky, but it's certainly doable. In a browser, there's also a chat function, but I'm not sure if it's available on phones. Anyway, I hope I've helped and feel free to ask any questions if I've been unclear.