See Chromebook C720 review on ZDNet: Acer C720 Chromebook first impressions: Fast and cheap
I've owned the Acer Chromebook 720 for a long time compared to most gadgets I buy, and the hardware is getting very long in the tooth. The old Haswell Celeron processor has been lapped by newer models. Even with hardware that most would consider outdated, the beauty of Chrome OS is how well it works on lighter hardware.
This is a big advantage that Chrome OS has over OS X and Windows. No doubt some will find it to be a disadvantage as it has fewer features, but Chrome provides a lot of functionality for its light footprint. That's why it runs so well on old hardware.
In fact, the Chrome browser runs faster on this old Chromebook than on any other device or platform that I use. Actions are executed instantly, something that is too often not the case on other systems.
I like how I can run web apps in a window if I choose, while leaving the browser in a familiar tabbed layout. It's easy to have them both on the screen at once for easy reference.
The Chrome OS task manager is really useful when multiple windows are active. A 3-finger swipe up on the trackpad is all it takes to bring it up and move around the windows. It's kind of like the expose feature of OS X.
See related: 11 tips and tricks for the Chromebook
All of the tricks you can do with Chrome OS on Chromebooks like the speedy new Pixel you can do on every model, even old devices. Sure, they won't respond as quickly but in most cases they will be more than fast enough.
The new Chromebook Pixel is state-of-the-art hardware aimed at those wanting a premium laptop. Does anybody need it to get good results running Chrome OS? No, as anyone using an aging Chromebook like my C720 can tell you.