LOL- yeah - put it on gramma's PC and watch her surf the net- be sure to get a unix/linux/solarix (they are all the same) geek over to 'mount' her drives
Well, it's a server OS. Get Grandma a Mac. In fact, get ANYONE who doesn't need to share files with PC users a Mac. Windows XP is usable and pretty stable, but it still imposes too much of a knowledge burden on people who aren't computer-literate.
Did your mother ever have any children that lived?
Unix, Linux, and Solaris are not the same thing. Not by a long shot. That you even say so means you don't know what you are talking about. I won't fuel any flame wars by saying which is best. I will say I'd a Posix based OS before Windows.
I haven't seen the new Solaris yet, but plug and play is not new to Unices. Mounting drives is no larger a task in Linux than it is in windows.
If you do put Linux or Solaris on granny's computer, she will be far safer from worms and trojans than she is with Windows.