It's just okay, but it's no L4D. L4D is brilliant.
It's from Valve. I noticed from the start how much it feels like Half Life 2 which I love to pieces.

I play it coop on my 360.
1. WoW
2. HL2
Although I have played a long long string of Mac games since 1993, I currently have 2 Mac games installed- WoW, which I no longer play and is no longer updated and HL2 which I just downloaded to see how it works on my MBP. Starting up HL2 I am once again amazed at the brilliance of this game. If you are a Mac gamer who has not played HL2 I highly recommend the solo story. I'd also recommend the original HL (ideally it would be played first) but that is not available for the Mac. I don't know if it will be converted or not.
For your reference, HL2 is a continuation of the original HL story. In the original, you played Gordon Freeman a scientist at the Black Mesa Research facility that gets over run with aliens when a portal is accidentally opened up. Your goal becomes to escape the facility through tunnels, ducts, sewers, and vast underground and above ground facilities. It is a grand adventure and a feast for the eyes! At the end of HL, the invasion is not resolved and when I started HL2, I was floored at what had happened.
Spoiler: Earth was defeated by the aliens and you are now part of the resistance! :End Spoiler.
Regarding Mac Steam, I did not have enough space to install anything in my Documents folder as Mac Steam insists, so I used a symbolic link to put Steam content on my external drive. There is now a
MaRumors Guide that shows you how to do this.