There's an insightful post on CB Insights called "It's a Boom, Not a Bubble," written by Tony Tjan, CEO and Managing Director or Cue Ball Capital, and Andrew Fu, Associate at Cue Ball Capital, that compares the tech bubble of 2000 with today's environment.  The article is data-heavy (which I like) and concludes that things are different and that this is a boom, not a bubble.

Link to article: https://www.cbinsights.com/blog/tech-bubble-boom/

Mike Moritz, the Chairman of famed venture firm Sequoia Capital, has cautioned that valuations are very high and that a number of "unicorns" - startups that are valued at over $1 billion - will become extinct.  Both Business Insider and Fortune have good analyses of Moritz's warnings (the remarks were made originally in an interview with The Times of London, but the article is only available to subscribers).

Moritz joins other notable venture capitalists, such as Bill Gurley of Benchmark, in warning of valuation concerns in Silicon Valley.

While Moritz does believe that valuations are "very sporty" and that some unicorns will fail, he also adds that "a good number" of unicorns that will succeed.

Moritz also feels that while there will be a setback in Silicon Valley tech, he does not believe it will be as bad as the tech bubble bursting in 2000 because companies today are more sustainable.

Links:
Business Insider article:
http://www.businessinsider.com/michael-moritz-warns-that-the-bubble-is-about-to-burst-2015-3
Fortune article:
http://fortune.com/2015/03/24/sequoias-moritz-joins-chorus-of-concern-about-silicon-valley-valuations/

Bill Gurley warned of there being "no fear in Silicon Valley right now" at South by Southwest, according to this article by Fortune.  While he said that we may not be in a tech bubble, we are in a risk bubble.  He added that he thinks there will be some "dead unicorns" this year, a unicorn being a startup with a valuation in excess of $1 billion.  Gurley also warned that if the free flowing capital dries up, it could have a ripple effect that will affect more than money-losing startups.  It could affect San Francisco real estate prices as well as companies that rely on spending by these startups.

It's a good article and worth a read.

Link to Fortune article about Bill Gurley:
http://fortune.com/2015/03/15/bill-gurley-predicts-dead-unicorns-in-startup-land-this-year/

Link to Fortune article "The Age of Unicorns":  
http://fortune.com/2015/01/22/the-age-of-unicorns/

Link to Fortune's "The Unicorn List":
http://fortune.com/unicorns/

Mark Cuban has published a post on his Blog Maverick site called "Why This Tech Bubble is Worse Than the Tech Bubble of 2000", which is an interesting read, but one with which I generally have a different perspective.

The distinction Mr. Cuban makes is that in the Tech Bubble of 1999-2000 the tech companies were publicly traded, while today the tech companies (apps and small tech companies) are not.  Mr. Cuban contends that investors in public companies during the Tech Bubble had liquidity - the ability to sell their stocks on the public market.  Today, investors in these private companies have no liquidity, and so can't sell their stock if things turn south.  

A second point he makes is that there are significant numbers of "Angel" investors investing in these companies and he first states that he thinks most of the investments made by these angels are under water because of the lack of liquidity.  He then takes a jab at equity crowdfunding, the rules for which have not yet been finalized by the SEC, because he believes "there is no reason to believe that the SEC will be smart enough to create some form of liquidity for all those widows and orphans who will put their $5k into the dream only to realize they can't get any cash back when they need money to fix their car.

I respectfully disagree with much of Mr. Cuban's post.  First, he assumes that we are in a bubble today, while I think there is lots of disagreement on this point.  Please see my prior blog posts on this for more on whether we are in a bubble.  Second, as the law stands today, it is mainly accredited investors who invest in these private companies.  An accredited investor in the US is one who has net worth of $1 million (not including the value of their primary residence) or who has income of at least $200,000 each year for the last two years (or $300,000 together with their spouse if married).  This seems to contradict Mr. Cuban's comment about these Angel investors not knowing what they have gotten into.  Third, the Jumpstart Our Business Startups (JOBS) Act does provide for annual limits on investing through equity crowdfunding platforms.  The Act provides that people earning under $100,000 the investing limit is the greater of $2,000 or 5% of their income.  These limits suggest to me that the maximum losses incurred by "widows and orphans" should not be devastating.

I do agree that the lack of liquidity in private company securities means that it is harder to exit the investment, but my view is that sophisticated investors understand this already, and for unsophisticated investors there are protections in place already (and probably more to be included in the SEC rules) that will prevent them from betting the farm on a poor, illiquid investment.

Link to Mark Cuban's post:
http://blogmaverick.com/2015/03/04/why-this-tech-bubble-is-worse-than-the-tech-bubble-of-2000/

Links to prior posts on whether we are in a tech bubble:
http://www.allenlatta.com/allens-blog/are-we-in-a-venture-capital-bubble-more-thoughts
http://www.allenlatta.com/allens-blog/is-venture-capital-in-a-bubble
http://www.allenlatta.com/allens-blog/fred-wilson-on-the-bubble-question
http://www.allenlatta.com/allens-blog/tech-bubble-andreessen-and-conway-say-no
http://www.allenlatta.com/allens-blog/tech-bubble-two-views