As someone who also played this extensively on the PC side, the multiplayer setup on the mac side is very cumbersome. You download an application from a third party (gameranger) which handles hosting. This means everytime you want to play a multiplayer game, you exit borderlands, and launch gameranger instead, which means everytime you have a problem hosting, you have to exit, adjust settings, and relaunch the game. Worst of all, if you would like to chat with the people you are playing with, you have to pay for a premium account through gameranger. On top of that, Ive found the service to be rather spotty (today I spent an hour trying to join a game, launching borderlands, and continuously timing out). So if youre buying this game to play with friends, keep this in mind!
The actual game itself is a lot of fun. Very nice visuals (UDK engine with a touch of cell shading), with some very cool open world design. Occasionaly I find myself stopping and staring at the horizon, allowing myself to travel to the location I can barely see in the distance. There are some nice atmospheric effects as well to acheive the scope of the game. This game of the year edition really has a ton of content to play through, and the randomly generated weapon system is rather addicting. Im glad feral brought this game to mac, however the multiplayer experience (one of the best aspects of the game) has been somewhat ruined.