The perfectly balanced, multiplayer RPS...
First rule of Borderlands: Do NOT play alone, even though its still a blast. Multiplayer, preferably with trusted, non-loot-greedy friends is the most fun you can have on Xbox Live if you're a fan of RPGs. If you ever wanted to try World of Warcraft but thought it was boring, didn't want to pay monthly, or thought it was dorky (I'm guilty of all of these) this game is the perfect substitute. The controls are straight out of Call of Duty 4, which is by no means a bad thing, and navigation and aiming are both smooth and responsive.
Borderlands is a never-ending loot quest. With over 16 million gun configurations you will always find a better gun, no matter what you currently have equipped. Shields, which behave like those in Halo, also have an absurd amount of variations. You can also pick up variants of Grenade Mods, Class Mods, Artifacts and SDUs.
Grenade Mods: increase 'nade effectiveness, upping the explosive power or adding an ability such as teleportation, sticky or transfusion, which steal enemy health and give it to you.
Class Mods: give an extra boost to your stat tree skill points, sometimes offering extra Exp and Health or Ammo Regeneration.
Artifacts: add an elemental ability to your special attack.
SDU: storage deck upgrades up your inventory slots.
(Minor) Complaints: the trade system involves throwing weapons on the ground, which is a bit risky, and you can't share funds or ammo.
With 16 million randomized weapons and four playable classes that change the experience and how you play through the game, Borderlands has incredible replay value. Not to mention how entertaining it is to mow down a gang of Midget Bandits or to take down a demon-dog the size of an elephant with your friends. Definitely recommendable, even if its just to hold you over 'til November releases.