Borderlands tries to shine with it's multiplayer but falls just a tad short of the goal line. The game can be played with up to 3 other friends via the internet or system link and 2 people using split-screen. More players in the game means tougher enemies and that means more and better loot. Items are on a free-for-all system and there is no way to change it, so make sure you are playing with buddies.
Make sure you and all your friends are around the same level as the game can become extremely difficult for someone who is even 2 or 3 levels below everyone else. That said, when you play with your friends who are all around the same level it will be hard to find another game that does cooperative run and gun gameplay as well as Borderlands.