While it is useful to carefully listen to user's diagnosis of problems, I have seen too many techs put too much stock in the user's diagnosis and miss the real problem entirely. Don't forget to take the user's information into account, but always use YOUR trouble-shooting skills to verify the issue.

I also believe that it is best to gently explain the real problem (and solution) to the user, if their diagnosis turns out to be mistaken. Treat your users with respect and courtesy and they are more likely to treat you the same way in return.