Cheating online is best stamped out as fast as possible - if a particular server or game doesn't ban cheaters I won't go back.

But I also don't really get why you would cheat in a single player game at all. If it is a game you can play SP and then MP online, cheating at SP would prevent you learning enough to beat online opponents.

To me it's a part of my completion habit - I want to be able to complete the entire game myself. This means there are still some of the advanced challenges on Portal I haven't yet done, but I won't cheat to do them - that would defeat the entire purpose.