Is this film a serious film? I think not, at least mostly not. If you go into this film, expecting a serious, crime/noir drama. It's unlikely you'll finish it. It can be enjoyed, to some degree as a straight film. But you have to go along with all the extraordinary aspects.
Cage plays a self medicating NOLA police officer. Down on his luck, embroiled in gambling debts, attempting to catch a NOLA drug dealer. But this film has almost nothing to do with whether or not he actually catches the guy. It pretends to, but it's more about how Cage's character manages to juggle all of the broken pieces of his life, while still managing a case, while under the influence of whatever drug he can get his hands on, prescription or otherwise. Whether or not he solves the case, the details, these things are not really important.
This is Werner Herzog and Nick Cage doing whatever they want. The end result, is pure awesome in my opinion. Nick Cage waltzes through this film with insanity as his dance partner. But for you to enjoy that, you have to find the comedy in it all. The intentionality of it all. Moments which, would in another film simply be unintentionally awkward , are supposed to be that way in this film.
In that sense this film relies on your interpretation. For instance you could consider none of what goes on in this film intentional, and therefore just bad.
An example of what I'm talking about would be, when out of nowhere, the camera stops to watch some Alligators.. the Alligators have nothing to do with anything, they're just there, on the side of the road. Or when Cage interrogates/tortures an elderly lady by holding her oxygen hostage. He also hallucinates Iguanas at some point. The Iguanas are irrelevant.. but so is most of what Cage experiences. Does that sound hilarious and awesome to you? Your answer to that question could have a lot to do with whether or not this film is for you.
If you enjoy that sort of thing than you'll love this. If not.. I find it hard to believe anyone is middle of the road on this film.