While the shambly, LA weirdness sounds like it may be similar to Inherent Vice or Mulholland Drive, the vibe is not. The surrealism isn't in your face, not even a sequence straight out of The Wizard of Oz. The naturalism of the shooting style is, by contrast, even more unsettling.
By the time Sam does a celebratory jig connecting the map from an old cereal box with back issues of a Nintendo magazine, you actually feel sorry for him. He's another ghost wandering this eerie city, obsessed with the totems of the past. (A major party set piece is set at the Hollywood Forever Cemetery, a real place, where you blithe young things can carouse near the corpses of cultural icons.)
Under the Silver Lake is way too long, but it's such a big swing that it could easily have been another hour longer and still, in its own way, worked. There's a piggishness to the whole enterprise. This is a movie about young, white men who clearly think the world owes them everything. Sam is mostly sympathetic, but that's primarily because we're with him all the time, and Garfield's confusion mirrors our own, but glimpses of his assholishness aren't far off.