this game certainly doesn't need cheats to be appreciated but rockstar usually has great ones. problem is i have no idea how to activate them... do you have to beat the story at least ones to unlock them?
Unlimited Bullet Time - Get Gold Medals on every score attack level
One Hit Kill - Finish the game on Hard Core
Bullet Cam On Every Kill - Find Every Clue
Unlimited Pain Killers - Finish the Game on Hard
So far the only way I have seen to use these cheats is in replay mode and I don't think it saves your last checkpoint. You have to pick your last checkpoint and then check the cheats you want and play. I have been trying to Find Out if there is a way to start a new game or have your checkpoints saved with cheats, but have not found any answers to date.Hope that Helps!