For all of you wondering, this is a valid steam key so works with PC as well as Mac
I just bought another copy of this game as my old key got banned.
This is, and will probably be for the considerable future, the best call of duty, and one of the best multiplayer experiences available. The vanilla multiplayer is great, but get into pro mod servers and it becomes amazing. It's so balanced, and a real challenge unlike playing against a bunch of 10 year olds on consoles.
The single player campaign is also very well made. A great story and well varied gameplay, although just like all the other CoD games it's a bit on the short side. Still, most if not all of the multiplayer maps feature in the campaign, which ties the whole game together really well.
If you're getting bored of new games coming out and see this on sale, definitely buy it. You will not regret it. There is a reason this game is still 14.99 most of the time on steam.