I tend to be a bit more 'real' when I post feedback, saying what I need to say.

Read the skill tips before you buy runes and select items.

Infinity Edge provides no bonus of what you need at all. Sure it gives a little attack damage, and some minor crit, but that's not where your damage lies. Your damage is pretty much straight magic. I only purchase Black Cleaver to help my team shred armor down.
Attack Damage be damned, On-Hits ftw.

Nice build random guy :)

I highly recommend removing Last Whisper and replacing it with Madred's ^^

You don't need to pierce armor, you can just deal too much Magic damage to ignore instead! :D

Rooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooofl!

Very nice dude this is a good solid build +1

Before I say anything, I'd just like to say I've mained Kog'Maw since he was released. After that, I played my favorite Vampire. Over half of my games played are Kog'Maw/Vladimir. This being said,
Seeing that Koggy is free this week, I'm not going to actually take this seriously. Here's why.

• You don't get Madred's Bloodrazers in your core. I can't stress how bad this is.
• Wit's End is a better choice for MR, it provides you with better damage too.
• Infinity Edge? No thanks. Frozen Mallet over IE any day. You need the health, not the damage. Kog'Maw should almost never be built with a pure damage item, except for Black Cleaver. It makes him too easy to counter.
• Attack Speed barely caps 2.0, if even capping 2.0.
• IE rush on Kog? Really? You'll get shut down quicker than an Annie with a Mejai's in ranked.
  
  You have good intentions, just not the skill level for Kog'Maw. One of the only good things I see here is Exhaust/Flash. Good choice, my personal favorite to run on this bad ***. Not voting until this gets cleaned up. People playing Kog with this build will get shut down. Sorry.