I don't think there is a person alive who won't contend that Macs don't play games well. They did, a long time ago, but not any more.
This is nothing new. What is new is Steam on Mac. With its recent release, this argument has been revived. Again. I'd like to take a second to point out a simple fact:
PCs suck at games too!
Here's the key fact. A PC you buy off the shelf is no better at playing Portal than a Mac is. Both standard Macs and PCs include low-power discrete video cards, or run on integrated cards alone. This is nothing new! If you want to play games on a PC, you put in a powerful video card. If you want to play games on a Mac, you put in a powerful video card.
The problem is that Macs don't have the expansion options that PCs do. PC components almost always adhere to similar standards, so parts are interchangeable. Nearly every PC has at least one PCIe slot suitable for a video card. Macs don't.
So for the record: Macs suck at gaming about as much as PCs do. But you can upgrade the PC, making it suitable for gaming.
Oh, and I'm a PC gamer. Screw the Xbox, my PC can run circles around it. I'm primarily a Mac user though.