Read elsewhere on ffxiah that its +50 dmg a hit. Seems to decay with each hit by 1 until canceling itself at 1 add effect dmg. This can be overcome since the spell cost is only 24 and the recast is 30 secs, and you can re-write it. Still don't know whether it stacks with ws.
Regardless, it seems definitely worth using unless you have a dnc in party