I respect it for the genre it spawned; but, it's just not that great.
The gameplay has some problems. You can't switch through weapons very well which is really bad in a game where you have to just to survive. Sometimes the levels seem poorly laid out too, as you can end up ending up in the same place alot since the hallways all look the same.
Which brings me to the game's worst point: the graphics. Now I totally think that gameplay makes the game, to some extent. This game the textures are just so muddy and everything is so dark that it's hard to tell what your doing. I think this is one time the graphics really effect the gameplay.
The music is spooky and sounds pretty good, but the noises the monsters or your character make are really weak, and they don't really sound like anything. A strange grunt, at most.
DOOM is pretty long, and it should take you awhile to beat it. It's really hard too, but how fast your done with it will really depends on how much your going to play it or how good of a gamer you are.
In the end, DOOM will always be seen as the start of some great games, but, really, when you actually sit down and play the game, do you have fun? Not really, and that's what makes DOOM an overrated classic.