Clearly Cruz and Diaz are Cruise's projections of the same person - he's paranoid about Diaz only liking him for his money and sees the negatives in her, whilst he sees the +ves in Cruz. I think the whole film is an individual's (futile) journey to obtain absolute truths, hence the transitions between waking world, dream world [perhaps Radiohead's "Everything in It's Right Place" playing at the start of the film was key, no one on the streets when he gets in his Ferrari (a clean start?). Cruise stumbles through the film wondering why people like him - is it his money - his looks (hence the disfigurement in the crash). I can't remember the film that well, I saw it ages ago, but I think that was the main message I was left with. There's always a risk in these types of film that the director tries to over-complicate to add a veneer of sophistication and to deliberately lose the audience.
I'd recommend Richard Linklater's "Waking Life" as a perhaps better exporation of the subject.