Well, I really enjoyed his game, mainly because the scenery is just beautiful and breathtaking at times...parachuting off a cliff and having a stunning view of the ocean and the smaller islands, and the white sands...beautiful :).
You're basically a guy working for two different "gangs" as such. But they are not rival gangs. You eventually work your way up in their ranks to become 'top dog'. There is so much to do in this game, and it's a real change from GTA or Saints Row, which are around streets of America completing missions...I would say Just Cause is much different to those games.
There are countless number of side missions to complete for both gangs, and also secret packages that can be found, some of which can be pretty tricky, and then about 8 races you can complete and get a best time.
The only downfall to this game, is that things are too far apart. What I mean is...if you go to start a side mission, the guy tells you where you need to be, and even if you're travelling by car, it takes you AGES to get to the place you need to be. I'm telling you, this place is huge...there are some things that can take you about 20 minutes to get to your destination! However, I found this to be a nice game to play. Kept me quiet anyway! I managed to 100% this game, and it took me (in total) 38 hours to complete (i haven't calculated that myself, it tells you in your statistics).
This is a good game, to keep you busy. I would recommend this to anyone looking for a change :)