Been using it for my company files for about two months. Its working really well. Even had a few conflicts when two people edit the same file - DropBox just makes another copy of the file with your username and the date and you decide which ones to keep.
We don't use it as a backup, the dropbox IS the place where we work directly on the files, and because it is an actual local synchronised copy of the files (not some clever ftp mounted volume) it gets happily backed up by time machine just like any other files.
Gets 10 out of 5 stars from me.
