Pros:

- Strong early/mid/late game
- Great Ultimate with crazy range
- Good Chaser
- Tank-killer
- Legendary Skin is awesome

Cons:

- People will hate your Kill-Stealing
- Gets focused easily
- Item depending

Kogmaw is an interesting champion... and he really needs attack speed and i will tell you why. I play kogmaw because hes just fun to play and he has a really strong early,mid and a very good late game. if you want to know how to play and everything, just go ahead.

Attack speed runes are the best one for kogmaw because he just needs attack speed for Bio-Arcane Barrage it will do crazy damage and you can kill enemies in a few seconds.

Nothing much to say, just an standard DPS mastery build.

Your first big item HAS to be Madred's Bloodrazor because in combination with Bio-Arcane Barrage you will do crazy damage! followed by Malady and Wit's End and you have even more greater damage + attack speed of course. Frozen Mallet in late game for slowing down targets and gain more max HP and your last item should be Bloodthirster because i like the lifesteal combined with your attack speed.

Max out Bio-Arcane Barrage first because this is your basic harassement in the early game and you will do BIG and i mean RLY BIG damage in late game with this skill. Maxing out Void Ooze next followed by Caustic Spittle. the point why is just put 1 point in Caustic Spittle is because i just like the little attack speed bonus...

-Your Combo the early game will be:
Void Ooze + Bio-Arcane Barrage + Auto-attack

-Your Combo in the mid game (starts at 6) will be:
Void Ooze + Living Artillery + Bio-Arcane Barrage + Auto-attack (Repeat Combo)

you can keep harrassing your enemy with Living Artillery every 6 seconds (because it costs more mana before the 6 seconds).

(When you r 18 you can cast Caustic Spittle after Void Ooze )

If you get chased and you want to get away just turn on Ghost and cast Void Ooze on your chaser. you WILL get away.

If you r in teamfights always focus squishy targets like Katarina, Teemo, Twitch, Evelynn, Sona, Ashe, Soraka, Zilean and so on. Yes, you can kill tanks RLY easy, but you should always focus squishy targets, never focus tanks first.

I think Ghost and Exhaust are the best summoner spells cause you can slow and chase your targets. you can pick Ignite too if you want to, but thats just my opinion.