I never ruined a single game experience by cheating, but by spoilers it did happen on a few occasions, most of them being because I was never interested in the concerned game's endings nor story developments (if any), or because I never had actual time to play until the end. I can name Morrowind being the best example I know by heart. I loved Morrowind back then, but the problem for me, and perhaps many others at the time (2002'ish period) was that sandbox type games were fairly new to the market, even on PC, at least to that extent of grandeur and complexity (seriously, back then Morrowind blew way more asses around than Oblivion did in 2006). So I liked it, a lot, but the damn thing was so big, the world was virtually impossible to explore entirely, it was intimidating (to me anyway).
So I played a good 200+ hours in it, if not more, over quite a long period, trying out many mods, and doing all the side quests I could find, but I always avoided the main story missions on purpose because I knew that if I saw the ending even just one time, if I would learn of the story and character developments concerned in the main story line that I would eventually lose the desire, the drive to resume and perhaps create a new character and start again from zero, it was my fear and I wanted to avoid that at all cost, giving me the illusion to play in some sort of a "never ending story" game. But after some time, in around 2005 I believe, I stopped playing completely, lost interest anyway, seeing the ending or not was irrelevant by then. Eventually I decided to look on the web to see the ending. I ended up finding a video of some random guy playing the last 15 or so minutes of the game.
So I "spoiled" the ending of Morrowind without ever completing it myself, but it wasn't because I thought Morrowind was bad, it was just because I never had interest to reach the ending by myself, and because the game was just so big (think 2002 standards here, I know Oblivion was much bigger, but Oblivion actually fitted in its time period, Morrowind on the other hand was slightly in advance of its time in terms of 3D world size to explore as you wished, and comparing it to Daggerfall is unjust, Morrowind's world was way more complex and polished, but anyway, I could go on and on about that). It happened similarly in Oblivion, I didn't look for the ending on YouTube or anything like that, but after reading so many posts of so many members in the mods section of the official forums I eventually saw the inevitable spoilers without warnings in a post, so I basically knew about half of the ending without wanting to spoil it by myself.
I am glad, however, to say in all honesty and as far as memory allows me to remember, that I have never ever cheated in a game with the direct purpose to beat it the first time through because it would be boring or too difficult. I really don't think I've ever done that even once, and if it did happen I don't even remember about it anymore. When I cheat or temper with game files it's because I've beaten it already or because I am doing modifications for it (that's the case for me in Fallout 3 currently, and it was the case in Oblivion as well, especially when I had to test my changes, I often had to use God Mode and spawn many characters with full and different sets of items). I really have no issues with cheating for one's own amusement (off-line, for instance, or with the agreement of others if on-line, just "for fun" for example), but cheating to beat a game on the first time through? Never.