The first time through, I didn't use it that often. Now I'm using it all the time.
Actually, as to Demise, it's not necessary to use the Skyward Strike against him. It's just so unbelievably hard that almost everyone I know has had to use the Skyward Strike. It didn't even occur to me at first to use it against Demise, so instead of simply cancelling out his electric sword like I guess you were supposed to do, I had to go on the extreme defense, blocking his attacks so that I wouldn't get knocked back and jumping out of the way of the electric shocks, then running up and slashing away at him in the precious seconds before he charged up his sword again. Extremely difficult, and even with 18 hearts, 3 fully-upgraded health potions and an invincibility potion I still died 4 times before I finally eked out a victory with only like 4 hearts left. Hardest end boss of my life.