If the primary use for a computer is web browsing, watching video clips, listening to streaming music, etc., I see a lot of advantages from a security standpoint to the relatively inexpensive slew of Chromebooks available now. More tech aware folks may have privacy issues with Chromebook or lament its limitations as a "browser-only" platform, but the security model of Chromebooks is probably currently the best available for "mother-in-law" level users who don't have a tech saavy person around to help them when things get inevitably messed up.

If your "mother-in-law" screws up a Chromebook (well, besides physically damaging it) and something gets wonky, a quick reboot will restore the operating system to the way it should be, automatically. Same thing if it somehow gets infected with something--a quick reboot should ensure that you're starting with a clean machine to do your online banking and such. Updates happen automatically. When somethings gets screwed up, no need to know how to roll back the operating system to some "last known good" state or go digging up restore disks to get things working again.

That said, I haven't yet owned one, but I'm seriously considering buying one in the very near future to run one through its paces. You can even tweak one to make its underlying Linux operating system accessible and you get a Linux laptop, but that's for more adventurous folks and does deny you most of the automatic security advantages of Chromium, like secure boot.