I personally think it'd be a good item to have early on. I play Kennen mid and I think it's gonna be a great item. It's on hit-gain really makes you think about emphasizing last hitting instead of strictly harassing. And 20 AP for 310 is pretty decent, an Amplifying Tome is 435 for the same amount, so~
But it's not like you have to build Zhonya's right after you do your first items, you can just keep it [It being arm-guard] for awhile, like when you play an ADC you might buy Sheen and keep it for 5-10 minutes before actually buying the other components for Trinity Force.