The game does have a main campaign, and this quote taken from the website's about page, explains the beginning of the story.
The orders are simple: hunt down and execute a rogue agent that has stolen top-secret intelligence and millions in Agency cash. For agent Rico Rodriguez, the mission is anything but routine. The target is Tom Sheldon, Rico’s former commanding officer, mentor and friend; a man who knows Rico better than he knows himself.
Sheldon went dark while operating in the corrupt island nation of Panau. There, three rival gangs wage war in the streets while the ruthless dictator Baby Panay rules the island with an iron fist. With the resources of the Agency at his disposal, Rico must infiltrate the island and locate Sheldon, setting off a violent chain of events that will set Panau ablaze.
Now, UNLIKE GTA, you are given a sweet grappling hook that you can use to pull yourself anywhere it can reach, like the Hookshot in the Legend of Zelda series, except without the restriction it has with what it can hook onto. You can pull yourself directly onto a moving vehicle, climb up mountains and buildings with it, use it to pull enemies and objects towards you, and even better, you can tether two things together with it as well, by launching the other end of your cable to another object. This opens up many combinations of things you can do, like connecting an object to a car, and drive the car around with a dangerous wrecking ball dragging behind you, or tether an enemy onto a tall building and have them hang there, helpless, while you beat them up with your melee attack like a pinata. You also get a parachute to help with aerial traveling, which you can use right as you're reeling yourself in with your grappling hook to get you started from off the ground even. This grappling hook + parachute combination is a great means of getting around places in the huge environment of Panau.
The huge world map of Panau contains so many upgrade parts to collect, government properties to destroy, and other challenges across the map that it will keep you occupied for a VERY long time. I finished the main campaign missions after 46 hours of mostly exploring and collecting everything I can find towards completion of the game, and yet, I have only completed 40% of the game apparently. This game will definitely keep completionists busy for a long time.
Graphics for this game is amazing. Textures of the environment are pretty, you can see things that are very far away on the horizon, and seeing all the explosions you'll be causing while creating chaos is extremely satisfying. However, this does come with a price. If you plan on playing this on your computer, you may want to double check your graphics card to see whether if it is strong enough to handle this game. Just Cause 2 may be the most graphically demanding game I own on the PC, and although it's pretty, slowdowns are fairly frequent on Pugyuu, my gaming laptop. In fact, I ended up having to set the graphics quality reasonably low to reduce the slowdowns and frame rate drops. Even in cut scenes, the visuals often fail to catch up to the audio. So unless you have a dedicated gaming computer, you may want to consider getting this game on the PlayStation 3 or XBox 360 instead.
If freedom, exploration, collection, and destruction is your kind of favorite gameplay, then this game definitely offers a fun experience for you. I highly recommend this game. Now, the only thing this game's lacking are hookers. They DO have strippers though... Of course, if you JUST want strippers, you might enjoy Duke Nukem Forever more.