In the WSJ post "Uber's $18.2B Valuation Is a Head Scratcher" author Christopher Mims discusses how the "moat around Uber's services is incredibly shallow" and discusses how competition, a "frictionless market" and Uber's lack of owning its ecosystem present issues for the valuation. At the end of the article, which is worth a read, Mr. Mims takes a look at Uber's valuation and says that even if Uber captures 50% of the world taxi market in 5 years, the company would still be less than $18.2 billion. Here's the link to the article: http://blogs.wsj.com/digits/2014/06/06/commentary-ubers-18-2b-valuation-is-a-head-scratcher/
The Huffington Post article "Uber Worth $18 Billion Because Sure, Why Not?" written by Mark Gongloff skeptically compares the valuation of Uber to that of other companies, and states that at $18 billion, Uber is worth more than half the companies comprising the S&P 500 index, more than all of the personal real estate in Falls Church, Virginia, more than Alcoa, Clorox, Chipotle or Whole Foods. Here's the link: http://www.huffingtonpost.com/2014/06/06/uber-18-billion_n_5460748.html
Finally, in the San Jose Mercury News article "Uber valued at a whopping $18.2 billion," author Heather Somerville notes that Uber is now the most-highly valued private US venture-backed company (ahead of Airbnb and Dropbox at $10 billion, Palantir at $9 billion and Pnterest at $5 billion) and worth more than rental car companies Hertz (market cap of roughly $12 billion) and Avis ($6 billion). The article also explores some of the issues confronting Uber, including regulatory and legal issues. Here's the link:
To me, whether this valuation is correct or not (and only time will tell), it is another example of how frothy the market is for venture capital-backed technology companies in the "sharing economy" sector.