FWIW, here is one anecdote for you: I bought a new computer for my
parents. They live at another hemisphere from the one I live.
They have been using Ubuntu for ? 1.5 year now, and have had no real
problems. Sometimes they do have questions ("how to burn a DVD with
photos", "how to make a copy of a DVD with family photos"), we share
desktops (VNC), and I show how to get it done.
There have been small issues (just like they had when running XP
previously). The difference is that instead of bringing the computer to a
local shop, and paying them some bucks; I help them through VNC. (I know
how to run/fix/admin a linux box, but not Windows).
My points are:
1. Linux is suitable for a desktop system. However most people will need
it to be installed for them (as even if they could, they actually won't
bother installing an OS).
2. Desktops present issues. With Linux or WinXP. In that case, folks like
my parents need support assistance. Be the local shop, or the technically
(BTW, the local shop where the computer was bought does provide linux
support, if there is ever need for it.)