It hasn't withstood the test of time.
The graphics in Gears of War have become outdated and the colors used in the environments and characters blend in all together making it a real eye-irritant. For instance, in the first level almost everything you look at is gray, brown, or shades of both colors (nothing else). Games nowadays include destructible environments, a more interactive AI, better graphics, ect. It may just be that I'm new to the 360 and have only played new releases that i find this game so outdated.
Don't get me wrong though, if you play the game in co-op, it can be somewhat of an enjoyable experience. Play the game on insane and you may find yourself actually having fun.
Now for the story... The story seems like a bunch of guys decided they wanted to make a cool game so they included plenty of bad language, huge and muscularly deformed soldiers, and tons upon tons of aliens. The story is annoying and don't be surprised if you find yourself skipping all of the cut scenes.
Bottom line, rent it. If you absolutely love it then purchase it. The game is so short you can beat it in a day if you really want to.
Who knows, you may even like the online gameplay --that is if you enjoy people only using shotguns to kill you in 1 hit.