I have used it ONCE in Arena, and that was a joke trouncing of 2 shadow priests (shadow ward, spell stone? hahaha), due to my hunter friends freezing trap. (which i insist is better than snakes in arena, use trinket or risk fear?)
I'm in the 49 bracket and love when i can sorta hide off to the side and hit someone that the group isn't attacking yet with curses and then que up the soulfire : ) dead before they know it.
With a Mystical Skyfire diamond proc and 5 points into bane, it becomes a 2 second proc. Much higher damage than a SB, a ton less mana. That's when it's usefull.