OK, just got through it. I think that this mod is nothing short of a work of genius. It is absolutely superb. It starts off slow, subtle, creepy and relatively easy but by the end the battles are epic. There is always something new to see at every turn and right to the end there are new elements and tricks to be seen. I could go on and on about it but why bother? I wouldn't know where to stop. I think I covered it in my second sentence.
Do I have any gripes? Well, just one little gripe-ette really. Personally, I found some of the original Doom monsters and weapons, particularly the more sci-fi-tech style ones a bit out of place. The mod has an almost exclusively traditional old-style horror theme with lots of very organic feeling stuff. I just found the future-tech elements of the original Doom jarred with it slightly. Not enough to spoil it or anything and I'm prepared to accept it as being purely a matter of taste too.
Anyway, people don't usually start babbling on about Cacowards until much later in the year but, to me, this is an obvious candidate.
One very minor thing that should probably be corrected. On one of the maps (map02?) there is one of those structures with a skin texture stretched between two metal poles. Most of these are blocking but this particular one isn't. So, you can walk through it and, when you do, you can turn around and, from the back, there is no skin texture on the line. Sorry, I didn't take a screenshot or anything to help track it down.