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.
He is way overdue on the rent and makes zero effort in earning a dime to pay it. An out-of-work actress occasionally stops by for sex, but Sam's interest level is about the same as when he masturbates to Playboy. When he sees the pre-teen vandals who put sludge in his car door handle and drew penises on the hood, he doesn't shoo them off; he beats the snot out of them. Later, he has something of a meltdown about how much he hates the homeless for making him feel bad.
Garfield, still a movie star, is an unshaven mess, with a gravelly voice and completely detached air. You know a guy like this, trust me, and he's probably working on a screenplay, too. Mitchell has certainly tapped into a very real strain of young male obnoxiousness. Whether you want to spend a long movie in its company is up to you.