What is wrong with "It just works"? I have found that there are very few pieces of software out there (especially operating systems) that truly "just work" out of the box. Most software requires at least some setup, customization, and a period of futzing-around before a user becomes proficient at its use. Chrome OS does one thing, and it claims to do it very well. Until most software is incredibly easy to use, I think "it just works" is a very appropriate advertisement.

Every modern operating system that I'm aware of gains performance increases through software updates. That does not mean that every company that creates operating systems purposely makes their operating system slower to gain favor over time.