I've logged over 226 hours playing this game, and the vast majority of that time was with absolutely no particular mission or objective in mind; just pure random destruction/mayhem "chaos"/crazy fun! Go anywhere at anytime, drive anything anywhere you can get it to, blow up everything in site, do insane stunts that are obviously impossible in real life just for the heck of it, etc etc all in a rather large gorgeous map! Almost forgot to mention, INFINITE # of parachutes and grapple fun to be had! All-in-all, this game is incredibly fun and very replayable without ever really feeling the need to play back through the main missions (you're in "mercenary mode" once you complete the game, which basically means just go nuts doing your own thing). I went nuts doing my own thing almost from the very beginning.
Something to note is that, as with all games that work well with it, the beauty and coolness is infinitely multiplied when playing with Nvidia's 3D Vision 2!
In addition to all the built-in awesomeness, there are also plenty of crazy and useful mods, found here: http://www.justcause2mods.com/index.php,
to make it even more awesome to play. I particularly found the increased grapple range & speed mod to be extremely useful in making this game INCREDIBLE!
The only downside is the repetitive nature of the buildings, vehicles, characters, and foliage. Once you've covered about a 4th or less of the map, you've just about seen them all.