I used to play strict 100% legit, but then my world got corrupted. After that, I started cheating in some minor items like wood, cobble, etc etc. After that world got corrupted (yeah, I corrupt alot of worlds), I started to cheat in things on my new world that I would find underground because I hate caving and feel it makes the game more of a grind than gregtech does. So, that world was corrupted. I made a new one, but with more mods installed (gregtech, some other stuff I can't remember) and then I hooked up MV to my centrifuge and it exploded. The explosion was incredible, but I repaired it. Then, I used a wrath igniter on my obsidian portal in the Nether and got trapped, with no obsidian and no way of getting out (Water boils up instantly in the Nether). When I got back (by cheating in obsidian), I started playing with the wrath igniter some more. Then, my stirling engine quarry exploded from power overflow and overheating. I repaired that. Then, the direwolf20 pack came out and I decided to try it on a new world. I couldn't get a Gregtech download (because Greg only likes to give updated mods to the FTB team and not somewhere like... y'know, THE WIKI) so I dealt without it. On that brand new world, I cheated in alot of stuff, including not only stuff that I could get underground but things that I could go out and harvest myself (trees, souls, alot of other stuff). The game started to lose interest after that, and I went through a process of deleting 3 worlds manually because I got bored of them, and when I heard news of the Ultimate pack coming out, I started playing that, cheating in whatever I so desire.
Quite the snowball effect.
Stay true to your word of not cheating in any items whatsoever and you'll have alot more fun than I did.