Very very fun.
Graphically, I think this game is this year's equivalent of Assassin's Creed. Vehicles are finely detailed, light reflection is just great. And as for the martian scenery, it is probably the most beautiful that I've ever seen on a non-terrestrial planet in any video game.
It's an open world, so you can kind of choose which mission you want to take. You need to complete a certain quota, but that can be reached without too much hassle, as missions are fun if only because you can blow things up. Gunfights are delightful too, although you shouldn't exactly jump right in the middle of a dozen enemy soldiers. It's realistic enough to be challenging, but easy enough to be fun.
In conclusion, get this game. I was hesitant about buying this game, because from past experience, games which aren't often plastered across the front page don't usually turn out well. Boy was I wrong. Get this, now.