well i actually rented this game on saturday night and it's pretty fun. let me tell you a few things about it.
the campaigne is pretty cool and it's got an 8 player co-op option via xbox live.
the achievements must be earned, you dont get handed 1000 gamer points for running through the single player campaign. they force you to play all multiplayer types and be good to get achievments. as well as the single player campaign. currently after playing it twice i have only 50 gamerpoints. wich is like 5-6 achievements or something.
the multiplayer is unique because theres a factor most games (if any at all) dont have.. A.I. running around in the form of scorpians and raptors wich works out well for most maps.
the maps are kind of bland but not terrible. they are no cod4 map but they suffice for the type of gameplay.
and finally the weapons are really cool and they all have some sort of secondary fire/function like a flare gun or a silencer.
so pretty much id say go buy this game if you already have cod4, its worth 60 dollars but not if you can buy cod4 for the same price first.