Like was stated you are leveraging your risk as well as your gains. And there is the interest charged. Other than that I love it. It enables me to move in and out of positions as I want to and I can trade option spreads, which allows me to spread my risk around much further than a cash only would. I only take on 2-500$ risk per spread and since it's leveraged I can spread that around even further. I stick to the "trade small, trade often" philosophy so it works pretty good for me.