"fanboy" is a cop-out term that means that you aren't willing to accept that someone else has a different experience and opinions than yourself. By using the term "fanboy" you are very clearly signaling that you are discounting anything and everything the "fanboy" says. This is the textbook definition of close-minded.

Far more often, the person slinging the word "fanboy" is the person who refuses to accept new ideas, to accept that other people's differing opinions may be just as valid as their own. This is what close-minded is. And quite often, the person who is tarred with the word "fanboy" is in fact quite rational and has good reasons for their opinions, and may very well be open to differing opinions. But you'll never find out because you immediately labelled them as a "fanboy" and shut down the discussion.

And thus, the simple rule. Anyone who uses the word "fanboy" is not worth listening to.