After the buff, Kog'Maw is even more fun to play than before. If you're a pro (or atleast have some skill) with his ult from before the buff, hitting things will now be a dream.

This is my build, the way I build him at the time, and the runes i have available. There may be better rune choices, but I think these work really well :D

There are many ways to play Kog'Maw, this is only a little part of what he can do. I personally think this way is the most fun way though :P

I got these runes for jungling and tried them on Kog'Maw. The result were complete desolation all through the game. The armor penetration makes you undependant on buying any of it ingame, so you can focus on the items you really need. The cooldown reduction is also nice, though Kog'Maw has quite low cooldowns for starters, you may consider some other glyphs. The armor is (if not a must) a very nice buff since Kog'Maw is very squishy.

You can basically spec ANYTHING in the masteries (according to your build ofcourse).
I honestly haven't tried these masteries yet, but this is what I would have specced.

Again this is a personal choice, Kog'Maw is a very versatile champion, experiment much with him to find what you like best, in order to be the best :D

I almost never build the exact same every game, I build according to the opposing team and how much gold I get (like if I can afford Madred's Bloodrazor whenever I'm back I'll probably buy it because it is really a great item (espesially on Kog'Maw), but the reason I put it so late in the build is because of the cost).

Malady and Wit's End work so great together. Malady reduces the enemys Magic Resist and deals 20 dmg on hit, Wit's End gives you Magic Resist and deals 42 dmg on hit. So basically you're stealing Magic Resist while dealing 62 Magic Damage on each hit additional to your AD. Not to mention the 90% attack speed they grant you (Malady 50%, Wit's End 40%)
And if that's not enough, with your maxed out q you'll lower your enemys Magic Resist and Armor by 25.

When you get your Hextech Revolver you'll heal quite alot on auto attacks, and very much when you activate your W. And when you get full build (if you haven't won yet) you'll heal whatever you do. Almost every item in the build enchants the life steal/spell vamp and it's so fun to watch you HP bar just grow like (well I don't know what, but it'll grow, trust me).

When you're playing Kog'Maw, DO NOT LOSE YOUR COOL. That is the key to being the great Kog'Maw player. If a player flashes in in front of you, you can either flash away, or kill them. If you get a little fed (Kog'Maw is very easy to pick up kills with) you can probably take most champions 1vs1, you just have to hit them with all you got, and when they attempt to escape, you finish them off with your ult.

Because that's the thing with Kog'Maw; his range. Many players don't keep in mind the vastly long range on Kog'Maw, use this to your advantage! (actually this happens naturally after a couple of games)

So just stay a little back, let the tanks tank, and when you feel it, FEAST IT.