It's finally here!
Each map contains one single monster type. I especially enjoyed the first imp map, the shotgun troopers and chaingunners maps, the first demon map and the cacodemon one. There are 25 levels (Few monsters share more than one map) with lot's of action and interesting details and textures. I really enjoyed beating the crap out of this map!
At the same period I am (still) playing Community Chest 3. It's a huge megawad finished by various members of the doom community and each map is unique in detail and size (sometimes I am tired running around specific confusing maps though) and there are several good maps with great surprises either in design or gameplay. I might post some more screenshots and details when I finish playing this one. The screenshot below is from CC3 and not from 1MW, while above you can see a screenshot of the cacodemon only map with a very atmospheric archway.