I cheat when I want to do something that the game isn't really designed for and that is after a few normal playthroughs. For example, do a fast solo playthrough of the BG games. I just mod my character stats to max, pick a mage/thief/fight elf and off I go destroying everything.
I also sometimes max my stats in the Elder Scrolls games to try new stuff (mega spells you can't normally cast is a good example, well before Skyrim came along and remove spellcrafting *sniff*).
The only case of cheating to finish a game was Tachyon The Fringe, I couldn't finish the last mission without using god mode (funny part about that game, your character would call you on enabling cheating). I don't remember if I cheat to finish Risen though. I remember getting really tired that normal enemies would take ~5 minutes each to kill in the last act. I had no problem with defeating them, it was just too long and boring for my taste, but I don't remember if I just put the game on easy or if I upper my character stats.