After getting this spell from Midir, I tried using it in about 20 duels in the hollow arena. Just for comparsion, I could land the slow and predictable crystal souls spear about once every two duels, but I NEVER hit somebody with the charged version of this. That's not only because the very obvious animation, but also because this spell, especially the charged version, has some hitbox problems. Despite the charged projectile looking huge, its hitbox is actually about the same size of the uncharged version, so it's much smaller than it looks and most of all doesn't go nearly as far as it looks. Also the projectile cannot rollcatch, as its short size and fast movement speed make it really easy to roll through it. If you want to use this in pvp I wouldn't recommend charging it, unless you think your opponent is very incautious. It's a shame that such a good looking and hard to obtain spell is so unreliable :T
Long story short: In my opinion the spell looks great, deals great damage and has rather low FP cost, but its deceptively small hitbox makes it almost useless in pvp. I think it will only be usable in From extends its hitbox so that it can rollcatch reliably.