The Big Sleep lays down the structure that we all have grown accustomed to in the noir genre. It certainly doesn't seem original, but that's because many authors have copied Chandler's style.
The book picks up quick in the beginning, but then after the two murders are conveniently explained in the newspaper, the story slows down a bit. There is more to come, but it isn't clear as to what Marlowe is supposed to be doing. Chandler created a reliable narrator. Keeping track of the plot in the latter half of the book can be confusing, especially when attempting to keep all the characters straight. However, at the end everything was ironed out.
I enjoyed that the mystery wasn't perfectly solved at the end. Yes, Chandler did figure out for the most part all of the pieces to the puzzle, but as far as getting proper justice, it didn't happen. There wasn't a big trial and an imprisonment, it all ended up being hush-hush. Goes to show that Marlowe isn't trying to bring justice, but trying to do what he thinks is right, which may not be the right everyone else believes in.
All of the characters in the story have something wrong about them, and it paints a depressing picture of the world around us.