Finished the original levels last night. I'd rank this one on up there with Doom 1, and miles ahead of Doom 2. It's a fantastic game. I've owned the N64 version for a while now, but never made it that far into it due to having other games to play. I'm glad I finally spent the time on it that it deserves.
One thing that surprised me was how much of an upgrade the engine is over the original ones. The later levels especially. Doom 1 and 2 always look like big abstract spaces with no real apparent purpose, but Doom 64 has recognizable spaces. Things like libraries, castles, are chapels are more obvious, and they make the space you're in feel more real and less like just a level.
I assume a lot of it is just art design, but I have to imagine a lot is due to the engine being upgraded for the N64, and being able to support more detailed geometry and textures.