Actually, Fizz isn't op at all, I can destroy him if I'm on the opposing team; though I'll admit that he's really hard to chase down.
I always start with Rod of Ages to reduce his squishiness, then, depending on how the game is going, I'll either get Hextech Gunblade for the spell vamp, or Lich Bane for movement speed and attack. Depends on how often you're chasing people down. If you're having trouble killing someone, Void Staff is a good choice for the magic penetration and good AP. So, generally, I have a full out AP Fizz with spell vamp and magic penetration, and I can lane forever, so Iong as I'm careful when tanks come about.