How do you use cloak effectively?
As an infiltrator, you're deadliest at long range with a sniper rifle. I find it most effective to use the cloak (1) to safely peek out of cover to line up headshots with the extra damage bonus, and (2) to reposition if the enemies get too close. This tactic works great in multiplayer, where an infiltrator can solo pretty much any wave without a timed objective by kiting enemies around and only taking shots from a safe distance.
Cloaking provides a defensive benefit of being able to revive squad memebers without attracting fire.