I don't think the 5 billion dollar valuation is out of the ball park considering that they solve a problem that a lot of people encounter including my mother. Their solution is simple and elegant and seems to work. Last statistics I heard they had 25 million users [1] and growing, while I dont know how many of those users are active subscribers I can imagine the conversion ratio is much higher then normal freemium products.
While there has been a lot of anti-Dropbox news in the tech press lately, we need to remember we live in an information bubble that doesn't reach the normal user.
[1] http://techcrunch.com/2011/06/17/billion-dollar-valuatio-clu...