First of all, Kog'Maws first item should ALWAYS be Madred's Bloodrazer, this item along with W will melt literally anyone. Anything besides MB is a wasted item slot. Philosopher's Stone is also one of the worst items for what it does. Getting it for gold is a waste seeing as how it would take 12.5 minutes to even earn the money invested in it back. The attack speed runes also seem wasted seeing as how Mana Regen would really help Kog'Maw out more. I also wouldn't get Zeal. It seems like it would be more effective getting a brutalizer or another BF Sword with that gold.

Also for your summoner spells, you shouldn't ever have to take Heal. If you're in the position where you have to use it, chances are you're already dead. I normally take any combination of flash/ghost/ignite/teleport for Kog'Maw.

Why do you have a mastery point in exhaust when you do not list it as a used summoner spell in your build? You should be using the 21/0/9 mastery build with Kog'Maw, but it wouldn't be out of the question to go 9/0/21 with him.

As a con you have "Easy to fail" listed. I find it impossible to ever fail as AD Kog'Maw. He's the new Ezreal.