Yes, I know, I already wrote about it, but now I'm through (not at 100% but anyway), so I can make this a real review. But since I already wrote about Red Faction: Guerrilla, I will try to make this review a bit shorter since I can mostly stick with my first impression: The game is a really competent 3rd-person shooter with tons of side-missions and some strong story aspects that really motivate you to finish this game. It never gets in the way of the bombing and shooting action but it just adds the right amount of atmosphere to make you please "just 5 more minutes".
Just a short recap: The game takes place on Mars and you are part of rebellion about the EDF, an army that actually were more less the good guys in the 2nd game. 45 years the EDF has turned the red planet into a giant slave-operation and slowly but steadily a rebellion starts, which you become part of. That story is told through audio-communications and some rare video sequences (more would have been really nice) and has some nice plot twists which might surprise some.
That said, the story about the one of the factions on Mars, the Marauders, could have been a bit more complete. About half through the game you get some strong hints about them, but no real follow-ups that tie in with the previous games. I hope that's what I'm going to get when I buy the extension pack (note that I didn't use the word "if" there), which more or less acts as a prequel to the main plot.
The whole package also works in this regard that most of the side-missions are actually rather short, which helps you move from one to another without noticing that you spent again more time than you excepted on Mars. The story missions, on the other hand, can take up to 30 minutes and are less repetitive. Most of the time, you still have to bomb something, but the part to get there is always different.
Part of some of these missions is also some vehicle action: Sometimes your superiors want you to hijack a convoy, sometimes you get a mech-like work-machine to keep the EDF busy while your co-fighters clear some depots. These missions most of the time end in a big chaos (esp. if you're playing the game on easy which doesn't punish you for just sticking with blowing everything up around you) so it might be less entertaining for some.
As to be expected from an open-world game there are also tons of collectibles. Since you have a quite powerful enemy, one is the destruction of their supply containers (250), then there are IIRC 300 ore deposits to be found and finally around 30 radio tags, that tell a little side-story. Find all of this will probably prolong the game for about another 5-10 hours, if you want to find everything :-) During the campaign you can also destroy some high-profile building of the EDF to lower their influence in the area. At first this mostly comes down to you getting as close as possible and then setting charges. But around half through the game you get something rather nice, that helps taking down such structures from the distance. Sniping buildings is just great ;-)