It's rare when I am impressed over something Sony does, and I hadn't heard of this spell before yesterday, I fell in love.
It lasts just over the time it takes to kill a mob. (need to time it). Being a rogue and not liking to draw attention to myself I was hesitant for the first kill. I was critting about 300 non-melee left right and center...but so we're the other melees so I was still safe..)
I don't know of any downsides to it but would highly recommend all shammies getting it when they are old enough!!
cheers