Defensive nightmare to those battling it, especially if the trainer uses potions or it knows the move Rest (or any health restoring move). I don't want it to be a legendary though. I'm getting a little ticked off at the number of legendaries that were introduced in the fourth and fifth gens.
If anything, I thought Goruugo should have been Steel/Ghost. Ground is so random... Desukan would have been made better use of the typing though with it's great defense and if it had Levitate, but Ghost/Rock would with Levitate would have been good too. It still fits and makes a nice addition to SS teams with a Sp.def boost.
And yes, Ghost/Steel is arguably the best Defensive Type combination in the game (especially with Levitate...). I've been waiting for it for "Generations."