I finished Red Faction: Armageddon, and my final verdict is ... Well, it's basically Guerrilla without the sandbox part and more emphasis on story. That said, if you're expecting a masterpiece story, you're going to be disappointed. And I would suggest you probably expect too much.
The action is obviously still what it's all about, and ... it's the same concept as Guerrilla, but instead of surgically picking apart buildings, you're given an arsenal of WMDs and a playground of destruction. This game isn't about assault rifles and that sort of thing, it's about grenade launchers, black hole launchers, plasma launchers ... well, it's about launchers. There's one weapon that only hurts organic targets and saves the environment, which is handy, but ... you'll find yourself tearing Mars' infrastructure apart no matter what. Probably one of the best scenes in the game was me in a room with two enemies. It was a rather large room and I was blindly firing grenades. I did get the enemies, but by the time I killed the last one, there was literally nothing else left in the room but rubble. Awesome!
I'd say it's roughly on par with Guerrilla - better in some ways, worse in others. There are probably going to be some people who are alienated by the, err, aliens and the lack of sandbox mode, and there are probably some people that are going to enjoy this more.