Jsnydcsa's mother and father use it. As some may know, they're complete novices with computers (e.g. "The internet is broken" or writing down friends' and family members' email addresses in a paper address book (when they have Outlook)) and I've been holding their hands for nearly forever on everything from programming the time on the VCR to the present day.
With caveats similar to jerryss's, I give it 5 stars for 'ol mom and dad.
Not that I'm tooting Microsoft's horn here, but I think MS has an interest in keeping this program (a) simple and (b) up-to-date and working against the latest attacks, etc. People just blame "Microsoft" or "Bill Gates" (I know, they're both 70+ years old, give 'em a break) for everything that's wrong with a computer, MS doesn't need any more bad press, so keep the program working.
Plus, they both use gMail POP, so I think between Security Essentials and gMail's junk filters, they're pretty well covered.