Chrome OS has a bum rap that it is not a full OS. Chromebook users know that is not the case but for those who must run OS X, Windows, or Linux it can be insurmountable. That's no longer true on the Chromebook Pixel for those who need to run Linux. A simple process installs Ubuntu and it can run alongside Chrome OS. Once Ubuntu is running, a key combination can toggle between Linux and Chrome OS without missing a beat.
The simple Ubuntu install method is thanks to a script developed called Crouton. Once developer mode has been enabled on the Chromebook Pixel, Ubuntu can be installed and run at will. This gives Pixel owners access to the full complement of Linux apps, opening the Chromebook to a vast library of "real" programs.
It's important to know that entering developer mode will wipe your local storage so be sure to back up before you start the process. That can be done by simply dragging local files to the Google Drive. But as a Chromebook owner you already store everything in the cloud, right?
Once Ubuntu is running on the Pixel, switching from Chrome OS to the Ubuntu desktop/environment is as simple as hitting CTRL-ALT-REFRESH. Going back to Chrome OS is done by entering CTRL-ALT-BACK ARROW. The Refresh and Back Arrow keys are the ones on the top row of keys on the Pixel. Both Ubuntu and Chrome OS are running at the same time by sharing the Linux kernel of Chrome OS.
The first thing you'll have to do in your new Ubuntu environment is enlarge all the desktop icons and font sizes everywhere you can. Ubuntu takes full advantage of the high resolution display on the Pixel without scaling elements like Chrome OS. That makes everything in Ubuntu really tiny until you make things bigger wherever possible.
Why would you want to run Ubuntu? If the Chromebook is doing everything you need, then you don't need to run it. If you like to tinker with things, this is an easy project to do. However, if there are special tasks needed that you can't do on the Chromebook, odds are they can be done in Ubuntu. Bloggers may find Chrome OS to be perfect (as do I), but those who also podcast can run the Audacity sound editor in Ubuntu for example, something that isn't possible in Chrome OS.
Update: Forgot to mention that those who need to use Skype on the Chromebook Pixel can easily do so in Ubuntu.