Dark Eldar Special Characters aren't bad, but hardly anyone uses them. They aren't the "new batch" of SCs that don't require opponent's permission. And basically no one wants to be dependant on his opponent's mood for fielding them.
Also all SCs, except for Lilith, go against the Dark Eldar philosophy of getting many units cheap. Sure, if you have a nice opponent and points to spare, you can take one. But generally, you're better off spending those points on some other, probably more useful units.