It's like playing through a really badass action movie.
I'd have to agree with Jeff's review with respect to the gameplay: it's great fun. In Gears, unlike other shooters, it's all about taking cover, and blasting enemies whenever they give you an opening. The thrilling thing is that the AI knows this as well! Casual mode is what it's supposed to be: easy, while Hardcore mode is supposed to be hard, and it is hard. Multiple pathways give the game a bit of replay value, as well as achievement points which make you want to play through the campaign on different difficulty levels. The weapons, though a bit generic, are all usable. Some weapons, though, like the Torque Bow, are very difficult to use, and it takes a lot of practice to use those correctly.
Multiplayer is also quite a thrill. My only complaint with it, though, is that you don't respawn in a match once you get blown to bits by a shotgun or chainsawed to death.
Graphically, Gears of War looks really, really good. It reaches the point of almost-but-not-quite cinematicness. Plenty of hi-poly models with few animation glitches, and plenty of drool-worthy lighting areas and texturing make for a truly amazing graphical experience. But Gears doesn't escape graphical glitchiness. Some objects look like they're floating above the ground at times. And sometimes, when two players push the chainsaw button at the same time, both players get the chainsaw animation, but only one guy gets killed, leaving the other guy with flailing arms. It's very rare, but quite amusing!
Gears sounds excellent as well. The weapons sound almost exactly like weapons, and the soundtrack is not memorable, but is the equivalent to that of a movie.
My only main complaint with Gears of War is the story. It really isn't that good. Basically, Earth is being attacked by a deadly horde of underground aliens called the Locust Horde, which have waged a very successful war in wiping out humanity, and it's up to the surviving soldiers to stop it. You take command of Delta squad, and must fight against insurmountable odds. It's generic. But at least each member Delta Squad has his own memorable personality.
All in all, Gears of War is a pretty exciting experience. It's definitely worth a purchase in my book. Give it a shot. Or perhaps a saw-through.