Robinhood is the hot new way to play the stock market and the Silicon Valley tech startup has grown to 6 million users with a smartphone app, commission-free trades and marketing aimed at millennials.

This week, Robinhood announced a $323-million Series E financing at a $7.6 billion valuation in a round led by DST Global with participation from investors including Ribbit Capital, NEA, Sequoia, and Thrive Capital.

Jay-Z invested in Robinhood more than a year ago through Arrive, the venture-capital arm of his entertainment management company
Roc Nation. Other big-name Robinhood backers include Snoop Dogg and Nas, Techcrunch reported.

Robinhood said in a blog that it plans to use the funding to pursue its mission of democratizing finance for all.

But Robinhood is not for everyone, Wall Street Journal reported. It’s best for a small investor who makes frequent trades of a few shares at a time.

It says it makes money, in part, by sending customer orders to high-frequency traders in exchange for cash — a controversial but legal practice in the brokerage industry called payment for order flow.

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“The revenue we receive from these rebates helps us cover the costs of operating our business and allows us to offer commission-free trading,” Robinhood co-founder Vlad Tenev said in an Oct. 12 blog post.

That’s supposed to benefit individual investors because they can get better prices than they would at the New York Stock Exchange or Nasdaq.

But that’s only if the broker passes that benefit, which brokers call “price improvement,” down to the customer, WSJ reported.

For small trades, the benefits of saving money on commissions outweigh any gains from price improvement, said Chris Nagy, managing principal of KOR Group, a firm that studies the quality of trade execution.

If you’re trading hundreds or thousands of shares at a time, you’re better off elsewhere, said Nagy, a former TD Ameritrade executive, in a WSJ interview. You might pay a $4.95 commission at a rival brokerage, but you would save more than that because the shares would have a better price than at Robinhood.

“At the end of the day,” Nagy told WSJ, “there’s no such thing as a free lunch.”