As we have previously reported, Windows 10 is being offered as a free upgrade (for a year) to most users of Windows 7 and 8.
Currently, the people on these operating systems that have not yet upgraded to the new version will almost certainly have noticed various popups and notifications about the new operating system, with buttons to allow you to download and install immediately.
Microsoft will be taking this up a step in the new year. Windows 10 is going to be added into Windows's automatic updating system as a recommended update. This means that anyone with automatic updates switched on (which should be most people - this is the default and recommended settting to ensure Windows stays up to date and secure) will find that Windows 10 will be installed during part of this process.
While this is a logicial idea in theory (many people will be fine with the new version, but just not understand about the current need to opt in), anyone that is deliberately avoiding it for whatever reason will need to be wary.
As far as readiness of Windows 10 is concerned, we provided our own in depth look a few weeks ago. We have yet to find any big issues (beyond very old software sometimes not being compatible with the new OS)