I had an invite for the closed beta, went through all the steps to register and create an account, only to find they only supported Windows. What a waste of time.
I'm told they'll be support Mac and Linux eventually... but if your browser-based game requires yet another plugin to be installed, you've kinda missed the point imho.
I guess you could think of it that way. And it's probably true for most web applications...
But then you realize it's fucking Quake III. In all its glory, in your browser, against friends, and free. Yeah I'd install a plugin.
Do know this: almost all of the guys at id Software are Mac enthusiasts. Most of them prefer OSX over Windows, it's just that the current gaming market is still predominantly PCs.
There are also challenges with OSX that Apple hasn't made easy for game developers, so keep that in mind.