Kog'Maw is typically viewed as an AD Carry champ who excels in his late game.
However, I like to play him as a lane bully AP champ, who is great in both early game and late gaame. Simply poke with E when arcane comet is up and if possible, attack with Q and W. However, if you don't take manamune first, you may want to play a little safer, as you will run out of mana constantly using abilities, especially once you put a few points into E.

Pros:
+Great early game, great scaling into late game.
+Artillery: you can kill someone miles away!
+Good against both squishies and tanks
+Can be built as an AD champ if the team comp needs it
+Few counters in lane
+Easy to play

Cons:
-Falls behind if down kills
-No movement to escape ganks
-Not a great frontline, if someone gets
on top of you, you're probably dead.