It's like Grand Theft Auto IV on the Serengeti, but not really.
At first blush Far Cry 2 looks and behaves a bit like Grand Theft Auto IV: The game has an open world, friends who can come to your rescue, a phone you use to contact them, safe houses, a detailed map for guidance and a non-linear story line.
But where at its heart Grand Theft Auto IV is an action game, Far Cry 2 has its roots deeply embedded in the bedrock of shooters and from what I saw during my short time with the game it seems to remain true to those origins.
I asked the developers about the similarities and he seemed genuinely surprised, I think that's because most of these similarities are obvious design decisions made once you've decided you want to create a sandbox game and the parallels are for the most part quite superficial.
Friends for instance. In Grand Theft Auto IV, you can call them for help, but they don't really get involved in missions. In Far Cry 2 they are your salvation.