I'm about a week into this game and I have to say that I am really enjoying the co-op multiplayer. The single-player is uninspiring apart from the destruction technology... which is even more impressive than Red Faction: Guerilla. The story is about as cliche as sci-fi video games get, but isn't abysmally bad. I miss the sandbox setup immensely. The single player game may be a bit short and there is little replay value . As a guy who rarely playes a single-player game twice, I certainly didn't mind. But the destruction technology/physics engine is worth the price of admission and may be the single best gameplay mechanic I've seen in a console game in many years.
The multiplayer is a pleasant surprise. They threw away the competitive multiplayer from Red Faction: Guerilla, a very under-rated multiplayer in my book. Instead, they give you a "Horde" mode (for those familiar with Gears of War) where waves and waves of baddies come and your team of four is tasked with dispatching the minions of Mars. The weapons feel genuinely powerful and the gameplay can be downright frantic as collapsing buildings and flying alien parts fly about a generously large map. You level up and unlock certain weapons. My main gripe is that the multiplayer gets pretty easy if you have a good group of teammates. Completing certain maps allows you to unlock the "dark" versions of the same map, which make it tougher to complete. In some instances you are tasked with killing the enemies while protecting and rebuilding a main structure. It requires a bit of interesting strategy which hasn't been seen in many shooters.
P.S. There is a special unlock called "Mr. Toots" when you complete the main campaign. Mr. Toots is a baby unicorn that your character carries backwards and lifts his tail to shoot a stream of rainbows and stars to cause mass destruction. The sound and expression on the unicorn's face is, in a word, priceless.
http://www.youtube.com/watch?v=5GUk_Oe4y6k&feature=player_embedded