Good idea, bad realization.
The combat is way too easy. You won't feel big difference between weapons. Any SMG or MG type gun will get you through the game, you can even use basic pistols with infinite ammo, as long as you keep moving and shooting (and maybe tossing grenade or two) – you'll stay alive. You'll also be able to use many guns mounted on vehicles, but aiming uses auto-lock system, same as in consoles (it seems that SOMEONE has totally forgotten about thing called mouse).
There are plenty of things to do:
Side missions – taking over villages/houses or doing some small tasks. They are made really well and are quite different, but the advantages of completing are useless. Why do you need more safehouses? What you will do with better guns when you can beat the game with picked up SMG/MG guns from fallen enemies? The new cars… Well that seems only thing worth of attention or would seem if cars were a little more detailed and you could feel difference between pickup truck and cabriolet.
Races – it would be fun, it really would, but there is no sense of control in car.
Collectables – yes, there are plenty of "collectable missions", there is even logical story behind each mission, but the advantages are useless. Most of the time you won't bother picking up items near your location.
The graphics are really good; cutscenes (in terms of graphics) are excellent: you get the feeling of what San Esperito would look and feel in reality. Problem is that the whole island looks very similar and after a while you understand that one side of island is almost identical to the other one.
Just Cause had all the chances to become equal (or very near it) to GTA series, but the lack of efforts made it simply disappointing game.