First of all, the movie poster for this is awful, so don't let that subconsciously affect your opinion. In reality, the cinematography in this film is incredibly artful and beautiful to look at. The writing is also extremely advanced, which results in a certain level of difficulty understanding what is going on and what the characters are saying (concentration is a must), but I found it to be more of a challenge than an annoyance. At any rate, when you do understand, it's a lot of fun: the chemistry between Kilmer and Downey Jr. is fantastic, as is their acting, and the narrator breaks the fourth wall constantly which adds a lot to this film's originality and quirkiness. Thematically, this is a very dark film, but the humour and the characters end up making this more of a light-hearted affair. This is a great film to watch if you're looking for something intelligent and mature, but funny and original.
In a sentence:
Incredible cinematography, original and quirky writing and fantastic acting make this one of the best Noire films I've seen.