This post is for struggling shadow priests. I'm not looking to be flamed or debate every subtle nuance or usage of Mind Spike.
My advice would be not to use Mind Spike AT ALL. At best, its use is situational, at worst, it severely gimps you. Only real place I use it is on low hp adds (like Ripsnarl adds). I posed the question on Mind Spike on trade the other day. I got back all sorts of answers except the correct one. I got back that it should be cast 3x during Archangel, then MB, 3x, MB til Archangel falls off. Uh, wrong. I got back that it should be used as an opener 3x, MB, then VT, SWP, DP. Uh, wrong. MB sucks without any orbs up and doesn't proc empowered shadow. The old Shadow rotation is only slightly modified. It should be: VT (as tank charges in), SWP, DP, Shadow Fiend, Mind Flay 2x, Archangel, MB... then on to dot refreshing and keeping empowered shadow up. Thats it. I sometimes cast SWD right after DP while strafing to give me an extra GCD of movement to proc the shadows and for the mana return. Also, you should be strafing during all instant cast spells.
Also, in Heroics, Mind Control on trash pulls as often as possible. Its a great tool.