Great film
This is a great film, but I think many people approach it with a misunderstanding, which is forgivable, seeing as how strange a beast this film actually is.
This is a film by David Mamet, whose films are all very similar, despite being in many different genres. Whether his films are superficially "spy thrillers" "romantic comedies" or "heist" films, they are at their heart examinations of people who use the tools of acting and deception in their daily lives. Mamet's characters are most often deceiving others, but are also, in some way, deceiving themselves.
Many people on the boards here have criticized Val Kilmer's performance as wooden, or along those lines. But I think it's important to note that Val Kilmer is playing a character who "acts," who wears personalities like disguises. This is key to understanding the character –that he can become whomever he is ordered to become, but there will remain a level of artificiality to his actions.
Just as actors are often portrayed in film as being overwrought, caricatured, mannered, or wooden, the various personalities Val Kilmer portrays in "Spartan" are all highly mannered, allowing us to ultimately understand the layers of his personality, and who he really is.
Spartan is a highly complicated film, and the story is itself the least interesting part. The real gem of the film is Val Kilmer's multilayered, nuanced performance.