This game was a really nice surprise when I started playing it. I had played the demo for quite a while before deciding to buy it (it's well worth a download), I'd never played the first Just Cause but this game became quite addictive for a short while.
The map itself is absolutely huge, it includes a full-city, a number of army and air bases, plus loads of unique little villages each with their own destroyable items. JC2 offers a wide range of cars, bikes, planes and helicopters to make getting around that little easier. There's a small amount of weapons, but the focus tends to be on the grappling-hook provided at the start of the game. I've never seen a weapon like it in any other game, you can tie people/cars to planes and helicopters, swing around from tree-to-tree or simply use it like any other weapon.
There is a storyline, but players can go in and out of it really quickly if they prefer exploring.
The game is supported by Steam and there are lots of achievements to unlock. The game can get tiresome for a while but is really easy to pick up and play.