So we have three episodes, that is 27 maps, in various themes, and it seems in no particular order. There's quite a lot of new textures, and some new tree sprites that are more green and less burnt. I assume this is why it used to be an IWAD, as sprite replacements and doom.exe don't mix well.
Monster population is usually quite low, most maps don't go above 100 monsters and I don't think any go above 150. However they're often large monsters, crowds of barons are comparatively common, and ammunition is very limited as well so it can get quite frustrating. Often you'll find that in order to avoid completely running out of ammunition, extensive infighting, leaving monsters until the end, trying to catch them in groups for more efficient use of rocket splash damage, use of fists/chainsaw etc. is required.
The author has a penchant for missing out upper and lower textures above and below the more colourful animated flats. This might have seemed innovative at one time but to me it looked ugly, especially when it all goes to HOM when you see it at the wrong angle. See for example the jet engine on E2M4 (or whatever it's supposed to be, the rest of the map is a hot rocks cave map under a deep cyan sky, so it really could be anything)
The author also has a penchant for cinemas. E3M3, which I think is one of the best maps, is a cinema complex and surrounding town. E2M2 has a cinema at the end, although it's mostly a castle and lava map with a lot of crushing ceilings. This is old school incoherent theme mapping at its best. Other maps are castles, space stations, towns, caves (E1M7 is part town, part underground flooded cave - it's really good except for having no ammunition or health)
In summary it's a bunch of old school maps that show their age slightly but aren't too ugly and randomly textured. Gameplay is tight and can be frustrating (from pistol starts - it's easier if you carry ammo forward, obviously) If you like megawads of smallish maps with, how shall I put it, some "innovative" design, it's definitely worth a try.