I use cheats(in singleplayer games) only when Im starting to run tired of the game. Its the last desperate attempt at getting some extra entertainment before the game is thrown away. And some games, like GTA SA have some pretty amazing cheats.
Trainers are mostly lame though. When the challenge goes away, so does the fun.
Cheats barely exist anymore, and the ones that do, mostly for GTA games, are so uninspired and dull.