someone correct me if i'm wrong, but i believe the difference between dropbox and foldershare is this:
foldershare syncs directly from computer to computer, so both computers need to be turned on in order to sync.
dropbox syncs to their central server, the computers syncing do not need to be on.
i believe the ease of use benefit of the central server for dropbox also limits it to 2GB (at least for the free beta) whereas you can sync more files with foldershare.
also, dropbox keeps previous versions of a changed file and allows restoring from a previous version.
i could see either service having its + and -, although the UI looks more intuitive in dropbox.