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Thing with Corki is... well, if you suck at landing his rockets, then Corki pretty much fails...
Phosphorus bomb is a good nuke, but mana intensive and you'll probably get to use it once to twice in team fight.
Gatling gun is pretty awesome for DPS Corki (his AA is not that great like someone before said, though gatling gun helps quite a bit).
Valkyrie is mostly NOT used for damage but as a blink.
Also, his AA range is not really the best in the game.
With that said, his rockets are what makes or breaks your game. And with smallish hitbox, it takes some practice to land them on moving targets.
Overall, Corki is a great and very balanced hero if you are able to use his arsenal well.