I am invested only in VTSAX - Vanguard's total stock market fund.
I believe pretty strongly in this strategy. I own the top companies in the US economy, so I share in the general upward trend of the market, without any additional risk (beyond the normal market risks that are baked-in to the economy).
I also have exposure to the international markets, to the extent that these top US companies are invested in other countries. I wouldn't want any additional exposure beyond this.
I won't have the crazy returns from investing in just one company that has a banner year, but I also won't suffer if that company has a crappy year. I believe that investors who swing for the fences will strike out more often than not, and their average returns over the long-term will be lower than the market as a whole.
And then there are the fund expenses. The VTSAX beats many other investments, just on fees alone.
If you want some other professional opinions on this strategy, I recommend A Random Walk Down Wall Street, by Burton Malkiel.
http://a.co/gpSZiA0Jim Collins didn't invent the strategy, he just put in a format that reaches a new set of investors.