In Cataclysm, the talent of Inner Will underwent a big artistic change. The former floating skull Shadowy Apparition was removed. As Blizzard planned, the ability now does procs clones of you, instead. It seemed not to be able to work very well at the first glance. However, after being tried on the battlefield, the animation seemed to go much more smoothly. One of the major concerns is the tax these extra shadow priests could have on your graphics card.
For those who play on more dated machines and laptops and prefer 10-man raiding to 25-man raiding, better frame rates and performance can be obtained. Shadowy Apparition can seriously bog down your computer on a movement-heavy fight, even though you simply run a 5-man instance. Frankly, the proc rate on these things is just that generous. Players can fill a whole raid with such things. Blizzard should do more about the talent. Certainly things will get much better in the near future.