A core unit that better than almost all comparable elite units out there (Witch elves is the only good match though) while being cheaper? Sold. And Daemon players have the gall to call them overcosted.
Without the Herald, I'd say they're overcosted by a point or two. Gors with handweapons (which are widely regarded as being a point too expensive, though this is slowly changing with the release of more 8th books) are 66% of the cost, about as tough and with a similar or better damage output against most foes. Daemonettes seem to pay a lot for two stats (WS and I) that don't really matter that much, especially with today's chunkier units and step-up.