Definitely an acquired taste!
However, because this is such a radical departure from the traditional FPS (most are on-rails, linear experiences where the pacing is purposefully scripted to crank the adrenalin constantly) there are many who will think that this game is a failure. Indeed, measured against the classic FPS formula, this just doesn't cut it: the pacing is plodding and slow at best.
But it must be remembered that the whole point was to make something absolutely unique: a FPS shooter with the free roaming pace of GTA. Before considering purchasing this game, you MUST keep this in mind. The single player campaign is NOT going to be COD4 in Africa. In addition to combining sandbox game elements, there are also some RPG customization elements as well. In the end, the experience amounts to something more along the lines of a second life or career, and less of a rollercoaster, adrenalin rush action movie. It is this very distinction which is at the heart of why you see this game getting 9's from some, and 1's from others.
But I will say to those who do find the game's style appealing: even measured against what the game attempts to do, it is still far from perfect. Just like GTA4, it lacks enough depth to make the sandbox experience empty and unsatisfying. It is still an amazing thrill however, to travel across these amazing African landscapes, but one can't help but feel that they are still just a bit too empty and lifeless to truly be realistic, convincing and interesting in the long run.