Looks interesting.
It's funny that their model mimics so many others in tech. Free now and they'll figure out how to make money later.
I think this will eventually fail. I guess if you're an active trader and paying a high commission now, it might be worth experimenting with.
They're backed by Andreessen Horowitz (Facebook, Twitter, Instagram, Pinterest, Skype and countless others) and Google Ventures, so I suspect they have a very good plan on how they will make money (it sounds like initially via renumeration and margin). I doubt many founders with their background and experience would waste years of their life on a hope and a prayer when they could be making millions developing high frequency trading algos. My guess is their strategy is to take significant market share from any other discount brokerages currently charging fees and provide an API so that other developers can build great experiences on top of it.
I don't think free is a stretch. I recall when $25 trades were the standard and the first brokerages came out offering $10 trades. People were skeptical and wondered if they would be getting a degraded experience. In the travel industry, I recall when paying $10 booking fees was the standard. People no longer pay the booking fees and the business model is the same (renumeration from the GDSs). Technology improves, software continues to eat the world, and as a result costs get driven down for all of us lucky consumers.
Will they make it? Who knows. However, I bet they have a pretty good plan in place and it will be interesting to see how it plays out.