Been using this for about a month in combination with the documents to go app on the iPhone and it has genuinely been brilliant for my needs. Though, I'm anxious to see what the new Apple iCloud service is going to be (i'm guessing a rebrand of mobile me and still a pay service).
But on with Dropbox, recently been using this in collaboration with an editor on a very long document. It's been great to just have changes made or suggested and do it wherever I am (nothing to revolutionary, but still, doing this on the iPhone is pretty nice). The old fashioned way, prior to this month, was emailing back and forth the document with changes, saving, editing, resending, etc.
In combination with Documents To Go (can create and edit MS office suite on iphone/ipad, other apps available, but seemed best value and most recommended) it was literally a text as I'm on the bus to alert me to comments, additions or revisions and I can check, add, etc there and then and we can both work on the document.
For me, this really is a perfect way of operating.