The only time a rogue should be using Feint is on Bloodboil, if he's not too sure of his tank's capabilities (and even then, you'll probably have a tranquil air totem).

How much does feint reduce threat?

When you get good weapons, Feint will become part of your rotation, think about that... every 10 seconds, for 20 energy you remove over 1 thousand threat from the mob. That's amazing, that means you can deal more damage without pulling aggro.

What does Cloak of Shadows not remove?

Instantly removes all existing harmful spell effects and increases your chance to resist all spells by 90% for 5 sec. Does not remove effects that prevent you from using Cloak of Shadows.