"Nightmarish" is probably a little bit of a lazy crutch-word when discussing horror movies, particularly those of the Italian variety. Who cares about narrative coherence, acting, or script quality (or paying enough attention to properly assess same) in the context of a nightmare? But even if that's true for all of Dario Argento's other movies, there's really no other word that can be used to accurately describe Inferno.
Much like a dream, the ostensible protagonist of Inferno shifts back and forth without any reason or narrative logic (no matter how confusing Suspiria or Deep Red get, at least the audience only has one main character to follow). Buildings will burst into flame for almost no reason - one late scene had me wondering if perhaps this movie was an influence on the Coens when they made Barton Fink. Scenes also shift, sometimes to the confusion of the characters ("where was I last night?"), and sometimes a simple trip to drown a sackful of cats takes a sinister and unexpected turn.
On the level of pure craft, Inferno is up there with all the best of Argento. His colors have never been more vivid, as his famously expressive camerawork is a tad restrained as if to compensate. And his set-pieces are as scary and suspenseful as ever - I, for one, will never be able to go swimming in a stagnant pool with a flayed corpse again without feeling a little bit creeped out.
The score by Keith Emerson doesn't quite fill the Goblin-shaped hole in the movie, but it's still a perfect fit for the uber-Argento material. The main theme, which build to its full intensity at the climax of the movie (go figure), is one of my favorites.
I can see a lot of people being disappointed by Inferno, because even in the rare stretches in which the plot makes sense it never quite crosses into "engaging." But if you go in with a mindset of appreciating dark surrealism than an a big roller coaster, you might find that it hits the spot.