I think I resented this movie when I first saw it because all these critics insisted it was a masterpiece, but it actually really is very good. It divides into four sequences: set up, the murder, fall out of murder, and the pressure coming in.
It's clear Neff isn't so attracted by Phyllis - though he does find her attractive - but by the desire to pull off the caper, and outsmart his boss, Keyes. The Neff-Keyes relationship is the heart of it. It's not too shocking that Phyllis is dodgy - she suggests killing her husband early on, and doesn't really do that much bad stuff to Neff (she insists they still make the insurance claim, she sees the Italian guy on the side... so what? Neff sees Louisa). But Neff genuinely betrays Keyes, who believes in him - offering him a job, but also defending him to his bosses. And the last scene is between Neff and Keyes.
Keyes has the best dialogue too - flowery, but rich and imaginative. The story is well structured, with good suspense moments (eg Keyes visiting Neff when Phyllis is there, the drunk man appearing at the train carriage). I will say though that it was a very convoluted murder plan - killing someone in a car (in a way to make it look like an accident too... was it choking? A head injury? Wouldn't those things show up?)