I was wondering a bit about this and did some calculations on this, and I've found that the effect of berserker's cry is a lot stronger than is shown.
Here's the strength of the skill my champ currently has.
Stats of the character about to get hit by berserker's cry
Stats after getting hit by the effect.
If we look at what the skill should do, then it should've downed attack power by 15%+150. Coming from 1693, this would've been an effect of 253 (15% of 1693)+150, so a -403ap total. This should bring my character's ap to 1290.
However, as we can see, this is not the case at all. Somehow my attack power has been lowered to 333, which is a drop of 1360 attack power. No matter how we look at it, that shouldn't even be possible.