I play GTA IV w/cheats after completing the story missions. there's hours of fun to be found dropping out of the sky in a car right on top of a hot dog stand and being blown into the river. Or seeing how many people you can run over in 30 seconds.
In GTA, I tend to use cheats if I just wanna screw around for a while, but not until I have already beaten the game. I found when I was younger that even though it's tempting during the story, it ultimately will spoil my dinner, so to speak. Now we have trophy support, so even if I hadn't have come to that conclusion a few years ago, it would have more than likely made up my mind for me.