Drew Houston the cofounder and CEO of Dropbox:
Relatively early on we brought a handful of people off the street (literally) for our first round of usability tests. Most of them had trouble even getting through the installer: for example, zero of the five people noticed that we had a tray icon (our most important UI element), and more than one person tried clicking the screenshots in the tour. This was a mortifying experience for us, causing us to add a Giant Ass Blue Bouncing Arrow pointing to the tray icon during install, and to tweak the coloring of the screenshots to distinguish them from your OS chrome. Forcing these people through some kind of folder mapping scheme would have been a colossal train wreck.
His entire response is great and worth the read for anybody thinking about usability issues.