Isn't the OS already available for free usage? Unless they make a premium version or something, it's not worth it.
Chromium OS (what you are referring to) is to Chrome OS what Cr-48 is to Chromebook. It is an open-source project that's meant for testing new updates before releasing them to the public. In other words, it's in constant beta and not nearly as stable as what Chrome OS will be once Chromebooks begin shipping out. You can download Chromium OS, but it's a lot more work than downloading an executable file and clicking "install." Unless you'r a developer I don't see the point in doing this.
I don't know a whole lot about Chromium myself, this is just my recommendation.