Yeah, it is pretty powerful. A harpy's song at least grants a saving throw every round, though. The dryad and vampire versions of charm don't.
The trick is that it is against EVERY harpy, so depending on the DM, that's 1-12 saves. Making a save only makes you immune to that harpy, not all of them. Only one can influence you at a time, but when you make your save against that one you immediately have to save against another. The DC is low, 11, but this can still be a brutal encounter. I had my guys up against 5 harpies and it scared the crap out of them, although ultimately once they made their saves and could reliably fight back it was a cakewalk, they ran the harpies off.
Unless I have misread it.