Just cause seemed really easy to get into and a lot of fun to me, whether you are doing the missions or just messing about.
It seems to be running on a pretty good engine too. A large seemless world with Motion blur, depth of feild, Realtime shadows, normal mapping and lighting, no oblivion style pausing or low detail terrain in the distance and it runs real fast. There is pop up, but thats pretty hard to avoid in games of this type.
On the bad side, i didnt like how invulnerable you are, the characters look bad, the animation is a bit dodgy and the lack of meaty sounds when crashing etc.
Overrall a pretty good blast.