Pros
+ High range
+ Good waveclear
+ Tank shredding potential
+ Decent burst
+ Fun to play!!


Cons
- No escape
- Needs to scale up

Kog'Maw's is his early game. For that reason you want to play him whenever you feel confident, that you can reach late game. You will at some point become unstoppable beast, since your ultimate has very long range and you can spam it to poke the enemy.
You don't pick Kog when you are against an assassin, an early game champion or healers like Soraka or Yuumi, because they negate your poke.

Don't use E if Manaflow is in cooldown, your E spends a lot of mana. Try to always hit the enemy and take some mana back
Use your R where you think the enemy will be instead of where the enemy actually is.