Horribly, horribly overrated. First off, I don't think in the slitest bit that he's a unique villain. I've seen this type all over the place, and frankly, that doesn't make him awesome. Also, he's far from fabulous, he's just an awful character. I never did, I don't think I will ever be interested in his personality.
I think he's really cool. The first time you meet him, he looked weird, yet Bad*** at the same time. The grab-your-sword move is also really awesome. The second time you meet he looks very... odd, let's just say. The fight was pretty hard, though. The third fight was flippin' epic. He looked like Fi on steroids, only he's a guy, and stabbing him from above before cutting his sword to pieces was really fun.
I found Ghirahim to be one of the best villains in the Legend of Zelda series. He was a little more creepy than past antagonists and by his various speeches to Link, he seemed very harsh and cruel which I enjoyed immensly. The battles with Ghirahim were all very fun in there own way, I personally like the third battle with Ghirahim the best. The first battle was a fantastic introduction to this great character and was a pretty big challenge for me at least. In the Fire Sanctuary, Ghirahim is the boss, this time he was a lot more difficult and fun to fight compared to the first fight. Finally, the third battle with Ghirahim was very unique in the way you had to push him off his platform and then have a pretty tough sword fight with him. The only complaint I have with Ghirahim is that I wished for him to appear more in the story inbetween his appearence in the Earth Temple to his battle in the Fire Sanctuary. Other than that minor complaint, I loved Ghirahim he was a very unique and original addition to the Legend of Zelda: Skyward Sword.