I bought a Mac because of the benefits that using a Mac brings.
Being predominately a console gamer, gaming sits quite far down my list of requirements and needs for any computer, although Steam being available and officially supported for Mac now is certainly alot more convenient. There are a handful of games, notably Team Fortress 2 and Counter-Strike, that I'm looking forward to playing with some of my friends now that the option is there. It's not something I'd go out of my way to do, such as "Maybe" acquiring a copy of Vista "if" my system was upto it, but now that it's here I intend on using it.
I just find some of the backlash against Steam for Mac quite silly, in some cases. No it doesn't support every game on the service yet and yes the service itself isn't working as well as it should but, hey, it's been out of beta for less than a week now. As has already been mentioned, Steam is only the store front. The future of what content works for Mac is upto the developers, either if porting their existing libraries or developing new software.
As a Mac and Playstation 3 owner, I'm just pleased that at least 1 of my systems is actually being supported by Valve now! Infact, even on my modest setup, Steam has been working fine for me. I've had no problems running Portal to a decent standard with some manipulation of the settings..