I love the existence of cheats in single player. Sometimes I've had a rough day and I don't want more challenge I just want fun. So depending on what game I'm playing I can either scale down the difficulty and feel like God of the wasteland (Fallout 3/vegas) or I can make the game silly by adding big heads, shooting paintballs etc.
I'm also not ashamed to admit that I will sometimes have been staring at a puzzle for hours before checking online to find that I just needed to hit a switch or something equally silly.
Multiplayer however is a different story. It's bad enough that due to reward systems a lot of multiplayer games are elitist and favour those who have been playing the longest even though those players are more likely to have memorised where everything spawns let alone having to deal with getting sniped from the whole map away through several walls.
p.s Right, Down, Left, Left, Down, Right, Right, Down, Left, X, Square, Triangle ~Ion Cannon!