The Windows beta was available a few months before the public stable version, and the stable versions for Mac and Linux are expected early next year.
The announcement on the Google Chrome Blog touts the speed of the Mac version, not only in rendering pages but in launching. According to the Google Mac Blog, "We also took great care to make Google Chrome a native application for Mac."
The Mac browser is fast, but it's still definitely a beta. Some very preliminary testing produced one page that wouldn't load at all on the first try (though it popped right up on the second) and another whose CSS broke (see screen shot).
On the Linux side, besides speed, the developers tout Chrome's integration with the "Linux ecosystem." For example, the browser can display in native Linux GTK themes, and the system package manager handles Chrome updates.
Both the Windows and Linux versions can also now use extensions, add-ons of the sort that are so popular with Firefox users. The offerings, in the words of the blog, include "cool, useful and cute extensions" -- examples (respectively) are an Aviary extension for taking a screen shot and editing it in the browser, the Xmarks bookmark synchronizer, and one that lets you choose your favorite Google doodle to be displayed with every search.
Mac Chrome extensions are promised "soon" for the Mac. The announcement says they're not beta-quality yet but that some will be available on a developer channel before long.