A fundamental question is how much leverage you want to use. You can easily bankrupt yourself, and unfortunately brokerages make this all too easy. I prefer using leverage with a loan to value ratio of 30%. For example, if I have $100k cash, I will use that to invest $130k in a diversified portfolio. The key is diversified, because you want to be able to ride out the downswings.
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People use leverage all of the time for home loans, and it is not unheard of to use a 80% loan to value ratio for a mortgage. For a $500k home, you might put up $100k cash, and take a $400k loan. You would likely do this at a reasonable interest rate, and your hope is the value of the home goes up faster than your interest charges. Your home value could fall, leaving you with a mortgage that is underwater, i.e. you owe more than the home is actually worth. All of this can apply to investing as well. Using a 30% LTV for investing seems perfectly reasonable to me.
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People use leverage all of the time for home loans, and it is not unheard of to use a 80% loan to value ratio for a mortgage. For a $500k home, you might put up $100k cash and take a $400k loan. You would likely do this at a reasonable interest rate, and your hope is the value of the home goes up faster than your interest charges. Your home value could fall, leaving you with a mortgage that is underwater, i.e. you owe more than the home is actually worth. All of this can apply to investing as well. Using a 30% LTV for investing seems perfectly reasonable to me.