Apple is
slightly different -- the licence for Mac OS X stipulates
that you're only allowed to run it on Apple-branded hardware. This is
somewhere between rare and unique, though, and it has recently been
relaxed slightly to permit use of hypervisors.
EULAs have the same value as toilet paper and should be used for the same
purpose.
Legally, they can and have been enforced. So their value is not nil
when it comes to screwing up someone.
I'd love to see more info on when one was successfully enforced on an
end-user.
Wikipedia is a good start.
ProCD, Inc. v. Zeidenberg
Microsoft v. Harmony Computers
Novell v. Network Trade Center
Ariz. Cartridge Remanufacturers Ass'n v. Lexmark Int'l, Inc.