First hour is fun. Everything after that will make you want to play Red Faction 1 instead.
The Good:
Great Graphics
GeoMod3 Destruction Engine
Well-sized Single-Player Campaign
The Bad:
Arcadish and disappointing gun play.
Very repetitive missions and very little diversity in environment, mission type, and enemy type. Often you are stuck doing the same thing over and over again, and fighting the same 3-4 kinds of troops that come out of the same 2-3 vehicles.
Relentless AI (even at the Green or Yellow warning level) that seem to come after you from all angles and respawn from places you don't think possible (For example, turning around, firing off a few rounds, turning back around only to find a squad of four enemies trying to melee you down).
The Verdict:
Keeping it short and sweet, Red Faction: Guerrilla brings a few new things to the table - namely the GeoMod3 Engine that has been re-vamped for the new game in the series. It also puts you back in the familiar setting of Mars, if you've played the first game. It has a decent selection of guns and a decent single player campaign, and yes, it is fun to blow the living daylights out of just about anything- and much satisfaction to be had from said destruction (rocket launcher v.s. building anyone? Or how about strapping a remote explosive charge to an enemy's back and watching him run around helplessly before you push the button?) BUT on the other hand, the gun play is very arcadish, it doesn't feel right shooting an enemy in the head and him not going down after the first, or even second shot. Furthermore the lack of diversity in the types of vehicles and npc's (be it your Red Faction comrades or the enemy EDF soldiers). The mission types are also very bland and repetitive, and will have you mainly just playing the game to acquire trophies - the real satisfaction comes from blowing stuff to kingdom come, and even still you will get bored of that after a few hours.
Don't fret though, multiplayer is where Red Faction: Guerrilla takes a step forward. Introducing special "backpacks" that give you unique abilities like vertically plowing through a building, taking off and flying for a short while, or healing your squad to name a few, the multiplayer game is quite fun. The destruction element really plays a factor in how you dispatch your enemies, and the trophies for the multiplayer portion are just as much in amount, if not more, than that of the single player campaign.
Okay so that wasn't short and sweet - but at least you get a basic idea. Watch the review on Gamespot, rent the game, see if you like it. My conclusion? Don't waste your money in actually purchasing the game unless you are a really hardcore TPS/Action gamer and a fan of the franchise.