I preferred the tense "sudden death" atmosphere that pervaded the AlienBreed-derived "doom clones" to doom itself, though the Doom games were enormous fun regardless.
It's a shame the sourcecode for AB3D2 wasn't a bit cleaner. I can't criticise Andy Clitheroe too harshly over it, he was pretty much working on the engine single-handedly.
As I discovered one summer, some of the editors were a bigger challenge than the game itself
The object and animation editors were particularly fussy, but nevertheless I managed to make a custom mod:
http://www.amiga.org/gallery/images/810/1_montage1_256.pnghttp://www.amiga.org/gallery/images/810/1_montage2_256.pngBuilding the custom weapon models and animations was fiddly. Unfortunately, the animation just uses a set number of frames, so on faster machines (read emulation), the animation becomes far too fast: for example, it's hard to see the grenade clip rotating as the launcher recoils. Pity the engine doesn't have built in tweening but what can you do?