I haven't finished many games since about the late 90s. By then my completion ratio was in a steep decline. I still play them but usually don't commit to them enough to finish them.
I was much more prone to finish games when I was a kid playing on consoles. The games were simpler, I had a limited library available, no internet, and I had nothing much more to do than homework and video games.
Every once in a while I can still get into a game that I play straight through, but it's the exception rather than the rule.
I hate to admit it but I have never played Doom or Doom II (or any of the counterpart re-releases) to completion. The only Doom game I have finished would be Doom3. Do any of you have any deep, dark, games that you have never played til completion?
I never owned the full version of DOOM, though with all the GOG stuff I've accumulated I might have it in there somewhere, I don't honestly know.
I had DOOM2, and I played legit up to the last level, but I never understood how you're supposed to beat it. I even looked in the level editor and it still didn't make sense what they had in mind. Eventually I used the 'noclip' cheat since I didn't know what else to do. So it wasn't legit, but I didn't really care. I was ready to be done and messing with WADs was more fun anyway.