Scream mask is an all around good buff and it's pretty underrated in my opinion. Bladers, Bowers, and anyone who hits multiple times in one attack gets screwed by it. If you just cast it on someone and a blader starts using chains on them, for instance, the blader always gets stunned after the first three hits of the chain, and the buff continues to interrupt them in a very annoying way. Too bad it only lasts 2 minutes like Pain Quota so it's easy to forget to cast it.
It used to be a lot better, though! Before the 90 cap update, you could get level 1 scream mask and it would work as well as a level 6 scream mask because stun was bugged that way. The worst part is that scream mask used to work both ways; not only did it stun when a buffed person got hit, but in addition, the buffed person had a probability of stunning the enemy with each hit.

Super, super cheap.