Kog'Maw is for me one of the best ad carrys in the game.
But sweet is that you can play him as an ap carry too.
When he starts his W he has one of the longest ad ranges in the game.
When you get ganked or want to come close to an enemy use your E.
And when you know that you got your opponent use your Q and DOWN.

Dont play to aggressive with this champ he is not mundo so armor and magic resis are not the highest fact that he can shows.

I have a friend whos plays the hole day Kog but only buy attack speed items but no DMG items.
Dont only but AS that is not good I said to him but he dont listen.
Three days after this I buy Kog and play him the hole time.......

I test items, taktiks ......win some games .......fail some games ........TROLL some games^^
Then I find a build that I liked (THIS BUILD ON THE TOP OF THIS SIDE).
I play a game with my friend with this new build to show him it.....and he was surprised.

And now he only play KOG'MAW with this build.

PROS:
-Good range
-nice attack speed and damage
-his ulti can observ bushes
-can hold enemis on range with his ult and his E

CONS:
-Low health
-his ulti cost more and more mana how often he use it
-not so good escape abilities

You can farm very good with Kog'Maw and dont be shy to use your W.
But you need this abilitie, because Kog'Maw is a late game champ and he need many last hits.
Also you need a support that holds enemies on range too.
But a big problem is that he has no really no crowd control.