It's an amazing film, one I personally found gripping the entire way through. Very French, though, and certainly a product of its time, but immensely historically important, both in terms of cinema at large, and in terms of telling a story that many people have never heard, and which should really not be forgotten in the modern age.
The wikipedia summary doesn't do it any real justice. It's like describing Star Wars as a movie in which a bunch of insurgents blow up a space station.
Costas Gavras films are about the human element, the fantastic dialogue and interactions between characters, and people as people. The events in and of themselves aren't the focus - it's what they tell you about society and humanity and just how mad and broken our modern world is. Films like Eden is West, Amen. (sic), and Z are about showing just how obviously absurd and wrong aspects of society and civilization are, how corruption flourishes and how evil always seems just one step ahead of good.
Z is dark, cynical, compelling, and tantalizingly situated on the brink of hilarity. So much of what happens in the story is just absurd, almost as if you were watching a farce, but with just enough of an edge and grounded so unmistakably in reality that you're caught between laughter and horror. As unbelieveable as the situations and events seem, you're constantly reminded that they're more true to life than we're comfortable realizing, and when you stop to think about things, you eventually have to admit they're accurate to life.