I had a really good time running through Sigil. These are the style of Doom levels I like with an emphasis on environmental puzzle solving and navigation, rather than strictly monster closets and ambushes.

Romero's brand of Doom feels less cheap and more interesting than levels designed by the other members of the original team and that personality comes through here.
This expansion focuses on switches you have to shoot to change the level, which works really well! There is engaging gameplay in searching for the switch you need to progress as well as in preparing yourself and a plan of escape for the level change and possible assault you will face when you shoot it. I am definitely not good enough at Doom to just blast through these in one try, but the trial and error and learning of the level was satisfying for my play style (failure and save-scumming).
Some things Romero loves to put in don't work quite so well for me. These levels have quite a few narrow walkways and ledges to navigate, which doesn't really feel hard or satisfying, just tedious.

Playing Doom is, of course, kinetic and even with the lack of a wider monster roster in this original game, feels great. The relatively low number of enemies lets the game focus on putting you in interesting situations and having interesting level exploration. Romero is obviously more interested in the level design levers he can pull here -- he only uses a couple of Cyberdemons and Spiderdemons in the whole set. It isn't about killing more and bigger demons, it is about finding your way through these spaces and exploring them completely. Approaching Doom level design in this way lets these levels feel unique and meaningful in a way that I find many levels in even Doom II don't achieve.

If you like Doom, Sigil is a cool set of expansion levels made by the man himself. Play it!