Underrated game, although with flaws
First off, this game went through developing problems, one of the biggest mistakes was probably switching the engine, when they had to recreate stuff from scratch, they had even particle effects in the quake 1 build, even though in software rendering yet back then, but perhaps if they just modified it like half life engine was, it would be better, but no, they decided to switch.
People call this Q2 ripoff, initially it isn't Q2 ripoff and I like the game better than Q2, which is for me not that great game as it is said to be and is overrated.
I really don't want to hurt Q2 fanboys, but by my opinion it is Q2 who ripped DK, in the early Quake 1 engine build I have played there was this level which was similar to that jail level of Q2, but it might be because some people left development and joined ID and back...
I am sick of people bashing this game and even saying lies about it.
The AI is screwed and the game has bugs, which started to appear once the original team left the developement and the coders started joining and leaving the game developement (and leaving mess) and perhaps Romero was being a bit lazier in the end. That's why alpha builds were rather stables and betas of this game were unstable.
I don't see this game worse than some really overrated games and it is a real blast in multiplayer, it has this special announcement system, different to Q3 or UT, it tells about what weapon killed who...if you kill someone with ion blaster the "announcer" will yell: "ionized!", or "made a mess" when killed by C4 launcher, "rode a rocket" by sidewinder and a lot more variants by each weapon. It adds a lot of pace to the game and paradoxically the multiplayer is the strongest part of this game.
DM is best and probably is the EP1 episode set too.
The least played and favourite part of game both single and multiplayer was Episode 4, it was rather too half-life like and not that fast paced as rest of the game. Not to mention this particular mission having only hitscan and melee enemies, which makes you load the game often and trying over again. Also the learning curve of this game is not that easy at first and it is fun to replay game and to find all the secrets and easter eggs too, I have even finished the game on hardest mode (which though makes you use your RPG skills the most).
Also patches fixed most of the most annoying bugs and removed the need for the savegems, but still few bugs remained (mainly singleplayer).
You can't save on moving objects for some reason, there is one level where if you save and load the position, it will break a sequence with a level boss and you will be unable to continue, fortunately this level is rather short, so it's not that much of problem to play it without saving. And episode4 bugs with sidekicks' weapons vanishing at one point (this seems to be patch specific bug sadly, but the rest of episodes are fixed), so you must lead them unarmed to get new weapons.
If you are fan of oldschool shooters and adrenaline pumping multiplayer and have open mind, you really should play this title, not to mention the music is excellent, although in mp3 format, which is known to have less quality than vorbis, or mod type tracks.