The monkey king is totally OP with his E. It basically does WAY too much dmg for what he can do. Here isa great post from another player talking more in depth about all the things he does.
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Monkey King has no sustain abilities. Staying in lane with him is very similar to staying in lane with Jax. You have to play safe, and drop clone when you pull minion aggro. Harassing with him will simultaneously push your lane, unless you manage to avoid using nimbus (E).
EDIT:: His passive isn't very noticeable. I haven't yet been in a situation where I know I survived something due to it. Most fights end very quickly with MK.
His Q deals insane damage, even without Sheen/Triforce. His attack range seems to lengthen a bit after casting it, so chasing for a last hit is pretty easy. Reduced armor just adds on to the "damage" you're dealing with this ability.
Clone is very useful, but without decent Movement speed, it won't get you a free escape very often unless you have dem pro juke skillz. the damage it deals is just a perk. Ability power scaling is moderately useful, but there's just no reason to build AP on him.
E is tremendously valuable for both farming and chasing, and does a sizable amount of damage throughout the entire game. The AD scaling needs to be toned down, otherwise this skill is simply too good for what it does, especially in teamfights, where he excels due to...
His !@#$%^&* ludicrous ultimate. Rank one has a base damage of 20, plus some of your AD. Rank jumps up to a base damage of !@#$%^&* 110. PER. !@#$%^&*. SECOND. This ultimate may very well become the reason for his bans in the future, and the constant QQ we'll likely see. Managing to land a tick of this ult on someone will deal damage, knock them up long enough to deal two more ticks of damage, and allow you to position somewhere for 1-2 MORE ticks. IN AN AOE.