Technically Robinhood is not free. There is a minuscule fee on all sell orders which is imposed by the US Securities & Exchange Commission. However, Robinhood does provide free stock buying options. There is a premium Robinhood 'Gold' account that also offers margins for added buying power and extended trading hours. Gold account monthly price depends on the tier you select.

Robinhood may not be suitable for Active Day Traders. No desktop trading platform. No short selling. Limited options in terms of securities. Lacks support for Mint [can't track your holdings outside]. Limited to individual taxable accounts.

I would recommend Robinhood if you are a casual mobile trader who loves to trade few stocks or ETFs per week. You may want to max out your tax-sheltered retirement accounts 401K/IRA before investing in Robinhood.