I have vague memories from 1993 of a friend showing the game first time for me. What stuck in my head were the Cacodemon and BFG, which he called the camera that kills everything. Also some memory of the Spider Mastermind and flashing it down with the BFG. I always thought it was the shareware until I released it must have been the full game because 1st episode didn't have Cacodemons, BFG's or Spiders for that matter.
My best memory of playing DOOM is from new year's eve 1995-1996 when we returned to the same friend's place where our parents celebrated the changing of the year. I was up in a dark room all by myself when everybody else was downstairs socializing and played through E1 (IDDQD on) and part of E2. I remember particularly well being absolutely lost in E2M2 for a very long time, just walking around and having imps rip and tear at my back without understanding where that noise was coming from because I had IDDQD on and shooting at anything that comes in front of me. Then before midnight we went to blow some fireworks outside and I didn't play DOOM until the summer when I got my own PC and started to practise what it is to design maps. :)
EDIT: Another great one was spending time at the summer cottage of my best friend and we played DOOM with pc speaker sounds on his fathers laptop, which was a 386 with 8mb RAM, eating salted liquorice in the same time. So much fun :D