I've successfully stopped 1 full blown exploit (admanteumcoin) where there was code that allowed a block to mine any amount of coins desired, (and had RPC calls modified to hide this).
I started out doing it to try to help the altcoin ecosystem, because it's pretty interesting, and because it's a great way to learn more about cryptocurrencies and all their implementations. My code review directory (that isn't actually up to date) is on github: https://github.com/Earlz/coinreviews