UAC_DEAD is a classic WAD. In many of the levels I have downloaded, the authors mention that they saw UAC_DEAD, or used ideas from it. I has received many good reviews, and I was looking forward to seeing it.
Almost from the start UAC_DEAD overwhelms you with its sheer size. Imagine an ordinary PWAD, but where someone accidentally turned the grid size up 2 steps while making it in Deu, and you get an idea of how huge everything is. Doors are 2-3 times their normal width (the texture repeats horizontally).
The starting courtyard is very impressive. A new sky texture has been imported, which is of a night sky - the author has wisely avoided the usual mistake people of making the stars too bright, and the starry sky adds great atmosphere to the outdoor areas of the level. You start inside what turns out to be an overturned truck; from outside you can see the wheels spinning, and the engine burning.
This level has many nice features like this. Some of the rooms are very impressive. There is a particularly good set of dark tunnels toward the end - the author has made the red walls flicker in the torchlight, giving a very spooky feel (this area is made more oppressive since it doesn't show up on the map - kind of a pain since the map is for areas like this).
There is also a wonderful room with a staircase in the air, which looks very impressive - this must be one of the earliest uses of special effects in a PWAD I think. The screenshot can not do this room justice - the flickering lights and detail have to be seen in action to be really appreciated.
However, many areas at this level suffer from being too large. The author struggles to fill the space in some of the rooms. There are lots of texture misalignments, though usually these are small compared with the huge rooms.
This level really breaks up into 3 phases in play. The first fight is good, as you battle with limited weapons against the many enemies in the main yard. You then find a large cache of ammo, a backpack, and a rocket launcher. Given these weapons, most of the rest of the level is relatively straight-forward; the rooms are mostly so large the opposition are swamped. For instance, one room contains half a dozen cacodemons, a dozen imps, and a baron - but the room before it is the size of a small aircraft hangar, so you have lots of room to dodge and get them fighting each other. Even so, there are still some exceptionally good fights, even if they are not too hard.
The last section of the level consists of fighting a cyberdemon and a couple of barons. The room is one of the smaller rooms at the level, but given the BFG and loads of cells there is plenty of space to kill the cyber. However, this area has a major bug, since the keycard is not made available when you kill the cyberdemon - this leaves you trapped and unable to complete the level. I have seen somewhere that this is a problem with later Doom versions. You will have to cheat to get the key.
Anyway, once you have killed the cyberdemon the level is pretty much over anyway. There is a very nice exit area, with a space shuttle on a runway; the area is virtually empty though, so is a bit of an anticlimax.
A truly classic PWAD.