Dark Forces was actually the first full FPS game that I played.
I got an Intel system pretty late, probably around 2000 or so, so I missed out on all the "classics". When I got my system, my friend put a bunch of demos on it, Half-Life Day 1, Jedi Knight, Heavy Gear II, etc. Before that, I'd played Doom for all of about 2 minutes and DF for maybe 5 minutes at a friend's house. (not because I didn't like them, but because I didn't want to be rude and hog his computer)
I loved the demo for Jedi Knight and immediately bought a copy off eBay, MOTS included, but I didn't want to play it until I'd played DF first. I finally got a copy, installed it on my W98 system and played through the whole game. I would have preferred true mouse control, like more modern games, but I ended up using a joystick in place of the mouse. I was never once bored or felt like I was wasting my time. After finishing it, I moved on to the later Jedi games, which I also enjoyed.
A couple years ago, I decided to play Doom/Doom 2. First I tried playing the originals, but they just seemed too primitive, plus they kept crashing on me. I then downloaded a copy of JDoom and used that to play through the games with full mouse control. Before I was even halfway though the first game, I started to get bored. All the levels started to look the same after a while. I eventually finished both games, but it started to become more of a chore than a game.
Recently I played through Heretic using JHeretic and I started to get the same feeling about halfway through. The novelty of the various creatures wears off and then it's just trudging through similar levels killing dozens of identical creatures.
I guess what I'm trying to say is that even with modern ports and full mouse control, Doom still gets boring, while DF holds my interest from start to finish.