I have to say this wasn't anything that I expect. While it is by definition a David Mamet film, it's a very different direction for him, and a bit of a next step from Heist.
The plot of this is a bit more complicated and not quite as self explanitory as other Hollywood fare, you're not quite clear on who's missing until about 15-20 minutes in. Mamet's trademark dialogue pops up from time to time, but this is more a film where dialogue is more about being real rather than being for the sake of the audience. Explanations are more drawn by yourself and by making sure that you pay attention to what's happening on screen, and making sure you understand the plot twists that come along the way (which are really well concieved, and add some unexpected turns).
Problem is, with the focus on realistic dialogue and a very solid plot, the characters in this film suffer immensely, and the actors aren't given much to work with. Val Kilmer I thought was alright in a role that is so focused on his goal, that there's not much about him we can learn about, other than he's spent a career in military and probably done a fair bit of black ops work. Same goes for William H. Macy and Ed O'Neil, who mark their presence, but really are more glorified cameos than anything else.
Spartan is not your typical Mamet flick and that will most likely disappoint the more faithful Mamet fans. He does action scenes with a very clean, direct and curt style, choosing not to let them become the centre of the film, which probably wouldn't be said in the hands of another Hollywood director. Plot is the reason to see this film, and characters, unfortunately suffer a distant second.