The devices we're talking about in this thread have all been Google Nexus products, not things locked down by a mobile telephone manufacturer or service provider. As a result the upgrades come at a fairly steady rate from Google.
I bought a Nexus 7 a while ago when I realised I would be without any of my computers for several days it has seen occasional use ever since. A while ago I heard about Android 4.2, and within a week of that a dialog popped up. Did I want to upgrade now, or would I prefer to do so in my own time? I was busy, so I picked later, and some hours later I remembered the offer, checked the relevant Settings page and updated.
One reboot later I was running Android 4.2. It was seamless, all my settings were left alone, everything was as I had left it, but now I had new features. One of the 3rd party apps I had broke due to some infelicity or other, it got an update later that week which fixed the incompatibility with 4.2, I would say that it was very like upgrading Fedora, except much faster and less scary.