In a word, wow.
Greater than the sum of its parts is what comes to mind. A well executed cover system and seamless animation make it very easy to buy into the whole thing as a reality. It's got all the grit, dirt, grime, and blood needed to create a very visceral experience.
I left the console world with the end of the Dreamcast. Gears Of War has for the first time since made me envious of 360 players for having already spent so much time with this gem of a game.
I don't intend to go into the gameplay details, that has been covered, covered, and covered again. I will simply describe it as a hybrid of Rainbow 6: Vegas and UT2k3. Imagine your playing UT2K3, take out the more fancifully weapons, add in the cover system used in R6:Vegas, slow it down by 300%, and add in even more gore and you will have Gears of War.
What this means is only the greatest multi-player action ever. We can hope anyway. I'm undecided on the small multi-player matches as of yet. Keeping them small has its advantages and disadvantages. It's hard to say if 64 player matches would make things better or worse without actually trying it. As it stands however the multi formula is great. Find some friends, find some enemies, and forget the next many hours.
If you have yet to play Gears Of War, you must do it now. Whatever you're doing, stop it, get to your nearest retailer and begin a new era of fun, ed... ness.