Borderlands is fun, but as John Funk pointed out in his review, the game doesn't really shine until you get into multiplayer. The shame of it is that on the PC the multiplayer system is broken, archaic, and shamefully lacking in features.
* It uses Gamespy. You know, like they used to do back in the late 90's, before people knew how to make a proper multiplayer platform.
* No servers, just a big list of anonymous people hosting games. You sort of grope around blindly for friends, or try your luck with random strangers.
* When someone activates a level transition, it pulls everyone else through with them. This opens up the opportunity for griefing.
* Some powers do team damage and some don't. It's actually possible for one player to be able to kill his teammates and for them to not have any ability to harm him in return.
* You can't ever kick anyone ever. Once they're in the game, you're stuck with them.
* If the host player disconnects, it dumps everyone back to the menu without saving anyone's progress, even though there is no reason not to.
* The voice chat? It's always on. Always. There is no way to mute it under any circumstances. I hope you don't ever want privacy.
* You can disable your mic from out of the game, though. Assuming you know where the config files are on your computer, and you know how to edit them and you know what settings to change.
This is just an overview of the brokenness that is multiplayer Borderlands on the PC. It's shocking how bad it is, given the polish and attention to detail we see in the rest of the game.