The keyword to summarize this E1 WAD would be 'consistent'. All the maps, save for M8, are identical in terms of aesthetics, size, and progression - and each took for me, who plays perhaps slower than average because i tend to explore (and managed to get generally 80% of the secrets), somewhere over 30min to complete. This isn't to say they are repetitive, as the same concepts are used in different ways.
The levels were more complex than original E1, but with the help of automap i never got lost, and i was never left wandering around not knowing what to do. It really is always a matter of getting the keys in order and progressing linearly, except for the fact that there is some necessary exploration as the layout of the maps does not, thankfully, follow that order.
Difficulty is somewhat above Doom. i died only once, on M1, because it is quite a long time before one gets a shotgun on HMP (unless you find the secret), and simply ran out of ammo in a critical place as i tried to pistol myself through Imps. On next attempt i made sure to use fists more.. At least on continuous play, the next levels didn't present this problem.
There are quite a few nods to the original levels, and the secrets are all in classic John Romero -style - which i liked a lot here.
There are few factors why i can't give this full five stars. There's sometimes an overuse of darkness and flashing lights, which is fine sometimes, but when overused, as in my opinion here, it just isn't any fun. That said, the maps are very atmospheric and frightening
Also, i did not like M8 at all. The idea is not bad, but there is for quite some time pitch darkness, and then a horde of Spectres is released in it, and only luck helped me survive - still i have no idea how it should have been handled. It seemed not only mean, but an exercise in outdoing the old Marquis in sadism just because one could.
Still, i found this set extremely enjoyable, and played it through in one day (HMP and continuous). My favourite map was definitely the secret level M9, which in style is not different from the others at all, but to my liking had the best layout. By way of comparison i could say that whilst i did enjoy DTWID as one of the first PWADs i played, this is much better (i haven't played enough of E1 replacements to say much more than that.)