Cheating against the computer when you've already had your fun the normal way is perfectly acceptable. However, if I can cheat against other people, I never will. I would feel like shit because I either cheated them out of a win, or cheated them out of their right to a fair game. I can't stand hackers online, because if you're going to cheat at least have it not harm anyone else. Also, I like the approach that newer games take when they disable achievements when cheats are activated, I just always found that cool.
I'm lucky seeing as how I've never encountered hackers, but I've had friends playing Diablo II lose their hardcore characters because some jackass entered the game with hacks and basically killed everyone (he was able to hit my friends even though they weren't marked for PvP). Luckily a GM was able to restore most of them, but others couldn't prove the guy who killed their character was hacking, so they didn't get their's back (it wasn't the same guy every time, and sometimes they were marked for PvP (no idea why you'd take that risk when you play harcore), but he was able to one shot them even though they were the same level.