Apparently you can't install Google Drive and point it to a non-empty folder.
And if you point it at an empty folder, then close the app and copy all the files in that are in your google drive, it creates them as duplicates, uploads them AND downloads the originals.
Whereas Dropbox just indexes whatever's in the folder you point it to.
I realise google drive is new, but good grief, this is a show stopper. I won't use it for more than a couple of hundred meg, because the hassle of doing all this again in a few months would get right on my tits.