In a fine article from Jakob Nielsen’s website, UseIt, Nielsen discusses the wisdom of making design decisions based on guesses and personal preference, versus basing them on data and facts. It makes for interesting reading. (thanks for the heads-up, Helen)
http://www.useit.com/alertbox/guesses-data.html
Summary:
Even the tiniest amount of empirical facts (say, observing 2 users) vastly improves the probability of making correct UI design decisions.
