After spending over 100 hours with this game (sp and mp), I can honestly say that this is the greatest game ever made.
What is the one thing that makes Gears stand out above the rest (Rainbow Six Vegas, COD3, GRAW, etc.)? To me, I'd say that its the feeling that you really have to work to get your kills. No easy 1-shot kills here (I'm talking to you RSV and GRAW). Yes, there's a sniper, but it takes a lot of skill to use and n00bs should steer-clear. Yes, there are grenade taggers, but try to come at me with one and I'll drop you before you get close. Everything in Gears just feels like there is a risk-versus-reward decision to make.
Is the game perfect? No, but it still leads to the best gaming experience, to date, on any platform.